feat: meter AI usage

JobTrackerApi/Controllers/AiUsageController.cs

@@ -0,0 +1,48 @@
+using JobTrackerApi.Data;
+using JobTrackerApi.Models;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+
+namespace JobTrackerApi.Controllers;
+
+[ApiController]
+[Route("api/ai/usage")]
+[Authorize(AuthenticationSchemes = "local")]
+public sealed class AiUsageController : ControllerBase
+{
+    private readonly UserManager<ApplicationUser> _users;
+    private readonly JobTrackerContext _db;
+
+    public AiUsageController(UserManager<ApplicationUser> users, JobTrackerContext db)
+    {
+        _users = users;
+        _db = db;
+    }
+
+    public sealed record UsagePeriodDto(int Calls, long InputCharacters, long OutputCharacters, long EstimatedTokens);
+    public sealed record UsageDto(UsagePeriodDto CurrentMonth, UsagePeriodDto AllTime);
+
+    [HttpGet]
+    public async Task<ActionResult<UsageDto>> Get(CancellationToken cancellationToken)
+    {
+        var user = await _users.GetUserAsync(User);
+        if (user is null) return Unauthorized();
+
+        var monthStart = new DateTimeOffset(DateTime.UtcNow.Year, DateTime.UtcNow.Month, 1, 0, 0, 0, TimeSpan.Zero);
+        return Ok(new UsageDto(
+            await SumAsync(_db.AiInteractions.Where(x => x.OwnerUserId == user.Id && x.CreatedAtUtc >= monthStart), cancellationToken),
+            await SumAsync(_db.AiInteractions.Where(x => x.OwnerUserId == user.Id), cancellationToken)));
+    }
+
+    private static async Task<UsagePeriodDto> SumAsync(IQueryable<AiInteraction> query, CancellationToken cancellationToken)
+    {
+        var totals = await query.GroupBy(_ => 1).Select(group => new UsagePeriodDto(
+            group.Count(),
+            group.Sum(x => (long)x.InputCharacterCount),
+            group.Sum(x => (long)x.OutputCharacterCount),
+            group.Sum(x => (long)x.EstimatedTokenCount))).FirstOrDefaultAsync(cancellationToken);
+        return totals ?? new UsagePeriodDto(0, 0, 0, 0);
+    }
+}

JobTrackerApi/Controllers/AiWorkspaceController.cs

@@ -26,7 +26,7 @@ public sealed class AiWorkspaceController : ControllerBase
     }
 
     public sealed record GenerateRequest(string Module, string? Mode, string? ExtraContext);
-    public sealed record InteractionDto(int Id, string Module, string? Mode, string Title, string Provider, JsonElement Result, DateTimeOffset CreatedAtUtc);
+    public sealed record InteractionDto(int Id, string Module, string? Mode, string Title, string Provider, JsonElement Result, int InputCharacterCount, int OutputCharacterCount, int EstimatedTokenCount, DateTimeOffset CreatedAtUtc);
 
     [HttpGet("modules")]
     public ActionResult<object> Modules() => Ok(new { modules = _workspace.Modules, provider = ResolveProvider() });
@@ -86,5 +86,5 @@ public sealed class AiWorkspaceController : ControllerBase
     private static InteractionDto ToDto(AiInteraction x) => new(
         x.Id, x.Module, x.Mode, x.Title, x.Provider,
         JsonSerializer.Deserialize<JsonElement>(string.IsNullOrWhiteSpace(x.ResultJson) ? "{}" : x.ResultJson),
-        x.CreatedAtUtc);
+        x.InputCharacterCount, x.OutputCharacterCount, x.EstimatedTokenCount, x.CreatedAtUtc);
 }

JobTrackerApi/Migrations/20260730203422_AddAiUsageMetering.cs

@@ -0,0 +1,23 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace JobTrackerApi.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddAiUsageMetering : Migration
+    {
+        // Intentionally a no-op: StartupInitializationExtensions owns idempotent SQLite/MariaDB
+        // column reconciliation and runs before EF migrations. The snapshot records the model change;
+        // the reconciler performs the provider-safe DDL without duplicate-column failures.
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+        }
+    }
+}

JobTrackerApi/Migrations/JobTrackerContextModelSnapshot.cs

@@ -26,6 +26,12 @@ namespace JobTrackerApi.Migrations
                     b.Property<DateTimeOffset>("CreatedAtUtc")
                         .HasColumnType("TEXT");
 
+                    b.Property<int>("EstimatedTokenCount")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<int>("InputCharacterCount")
+                        .HasColumnType("INTEGER");
+
                     b.Property<int>("JobApplicationId")
                         .HasColumnType("INTEGER");
 
@@ -37,6 +43,9 @@ namespace JobTrackerApi.Migrations
                         .HasMaxLength(64)
                         .HasColumnType("TEXT");
 
+                    b.Property<int>("OutputCharacterCount")
+                        .HasColumnType("INTEGER");
+
                     b.Property<string>("OwnerUserId")
                         .IsRequired()
                         .HasMaxLength(255)

JobTrackerApi/Services/AiWorkspaceService.cs

@@ -133,7 +133,8 @@ public sealed class AiWorkspaceService : IAiWorkspaceService
             _ => throw new ArgumentException($"Unknown AI module '{module}'."),
         };
 
-        var result = await _ai.SummarizeSectionAsync($"{instruction} {Guardrail}", source, max, 120);
+        var prompt = $"{instruction} {Guardrail}";
+        var result = await _ai.SummarizeSectionAsync(prompt, source, max, 120);
         if (string.IsNullOrWhiteSpace(result))
         {
             throw new AiUnavailableException("The AI service could not generate this right now. Please try again in a moment.");
@@ -148,6 +149,9 @@ public sealed class AiWorkspaceService : IAiWorkspaceService
             Title = title,
             Provider = string.IsNullOrWhiteSpace(provider) ? "ai-service" : provider,
             ResultJson = JsonSerializer.Serialize(new { text = result.Trim() }, Json),
+            InputCharacterCount = prompt.Length + source.Length,
+            OutputCharacterCount = result.Trim().Length,
+            EstimatedTokenCount = EstimateTokens(prompt.Length + source.Length + result.Trim().Length),
             CreatedAtUtc = DateTimeOffset.UtcNow,
         };
         _db.AiInteractions.Add(interaction);
@@ -174,6 +178,8 @@ public sealed class AiWorkspaceService : IAiWorkspaceService
         return true;
     }
 
+    internal static int EstimateTokens(int characterCount) => Math.Max(0, (characterCount + 3) / 4);
+
     private static string? NormalizeMode(string module, string? mode)
     {
         if (module != "cover-letter") return null;

JobTrackerApi/Services/StartupInitializationExtensions.cs

@@ -1016,10 +1016,16 @@ public static class StartupInitializationExtensions
             "Title" TEXT NOT NULL,
             "Provider" TEXT NOT NULL,
             "ResultJson" TEXT NOT NULL,
+            "InputCharacterCount" INTEGER NOT NULL DEFAULT 0,
+            "OutputCharacterCount" INTEGER NOT NULL DEFAULT 0,
+            "EstimatedTokenCount" INTEGER NOT NULL DEFAULT 0,
             "CreatedAtUtc" TEXT NOT NULL,
             CONSTRAINT "FK_AiInteractions_JobApplications_JobApplicationId" FOREIGN KEY ("JobApplicationId") REFERENCES "JobApplications" ("Id") ON DELETE CASCADE
         );
         """);
+                    EnsureColumn(c, "AiInteractions", "InputCharacterCount", "ALTER TABLE AiInteractions ADD COLUMN InputCharacterCount INTEGER NOT NULL DEFAULT 0;");
+                    EnsureColumn(c, "AiInteractions", "OutputCharacterCount", "ALTER TABLE AiInteractions ADD COLUMN OutputCharacterCount INTEGER NOT NULL DEFAULT 0;");
+                    EnsureColumn(c, "AiInteractions", "EstimatedTokenCount", "ALTER TABLE AiInteractions ADD COLUMN EstimatedTokenCount INTEGER NOT NULL DEFAULT 0;");
                     Exec(c, """CREATE INDEX IF NOT EXISTS "IX_AiInteractions_JobApplicationId" ON "AiInteractions" ("JobApplicationId");""");
                     Exec(c, """CREATE INDEX IF NOT EXISTS "IX_AiInteractions_Owner_Job_Module_Created" ON "AiInteractions" ("OwnerUserId", "JobApplicationId", "Module", "CreatedAtUtc");""");
                 }
@@ -1760,6 +1766,9 @@ public static class StartupInitializationExtensions
         `Title` varchar(255) NOT NULL,
         `Provider` varchar(100) NOT NULL,
         `ResultJson` longtext NOT NULL,
+        `InputCharacterCount` int NOT NULL DEFAULT 0,
+        `OutputCharacterCount` int NOT NULL DEFAULT 0,
+        `EstimatedTokenCount` int NOT NULL DEFAULT 0,
         `CreatedAtUtc` datetime(6) NOT NULL,
         PRIMARY KEY (`Id`),
         CONSTRAINT `FK_AiInteractions_JobApplications_JobApplicationId` FOREIGN KEY (`JobApplicationId`) REFERENCES `JobApplications` (`Id`) ON DELETE CASCADE
@@ -1842,6 +1851,9 @@ public static class StartupInitializationExtensions
                     EnsureMySqlAutoIncrementPrimaryKey(conn, "CvVariants", "Id");
                     EnsureMySqlAutoIncrementPrimaryKey(conn, "CvVariantVersions", "Id");
                     EnsureMySqlAutoIncrementPrimaryKey(conn, "AiInteractions", "Id");
+                    EnsureMySqlColumn(conn, "AiInteractions", "InputCharacterCount", "ALTER TABLE `AiInteractions` ADD COLUMN `InputCharacterCount` int NOT NULL DEFAULT 0;");
+                    EnsureMySqlColumn(conn, "AiInteractions", "OutputCharacterCount", "ALTER TABLE `AiInteractions` ADD COLUMN `OutputCharacterCount` int NOT NULL DEFAULT 0;");
+                    EnsureMySqlColumn(conn, "AiInteractions", "EstimatedTokenCount", "ALTER TABLE `AiInteractions` ADD COLUMN `EstimatedTokenCount` int NOT NULL DEFAULT 0;");
 
                     foreach (var (ixTable, ixName, ixColumns, ixUnique) in new[]
                     {