feat: meter AI usage

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;