feat(email): add Correspondence.Provider discriminator

b4 of the multi-provider email roadmap. The manual/free-text correspondence
entry path already existed (CorrespondenceController.Create) -- this slice
was narrower than the roadmap wording suggests: tag every Correspondence row
with which provider it came from (gmail | manual today; microsoft | imap
once those providers grow an import-into-Correspondence path of their own),
not build a new endpoint.

- Correspondence.Provider (nullable string), reconciled via the existing
  EnsureColumn pattern (SQLite + MySQL).
- Idempotent backfill: rows with an ExternalThreadId (historically only
  ever written by Gmail import) get 'gmail'; everything else gets 'manual'.
- GmailController.ImportSingleMessageAsync now tags Provider = "gmail".
- CorrespondenceController.Create now tags Provider = "manual".
- Both write sites use a fixed literal, not request input -- no injection
  surface introduced. Backfill SQL is static, no interpolation.

148/148 green (147 existing + 1 new CorrespondenceControllerTests; the
GmailController import test gained a Provider assertion in place).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

JobTrackerApi.Tests/CorrespondenceControllerTests.cs

@@ -0,0 +1,35 @@
+using JobTrackerApi.Controllers;
+using JobTrackerApi.Data;
+using JobTrackerApi.Models;
+using JobTrackerApi.Tests.TestSupport;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using Xunit;
+
+namespace JobTrackerApi.Tests;
+
+public sealed class CorrespondenceControllerTests
+{
+    [Fact]
+    public async Task Create_tags_manually_entered_correspondence_with_manual_provider()
+    {
+        await using var db = TestHostFactory.CreateInMemoryDb();
+        var company = new Company { Name = "Acme", OwnerUserId = "user-1" };
+        db.Companies.Add(company);
+        await db.SaveChangesAsync();
+
+        var job = new JobApplication { JobTitle = "Backend Developer", CompanyId = company.Id, OwnerUserId = "user-1" };
+        db.JobApplications.Add(job);
+        await db.SaveChangesAsync();
+
+        var controller = new CorrespondenceController(db);
+        var request = new CorrespondenceController.CreateCorrespondenceRequestV2(
+            job.Id, "Me", "Called to follow up.", "Follow-up call", "Call", null, "outbound", null, null, null, null, null, null);
+
+        var result = await controller.Create(request, CancellationToken.None);
+
+        Assert.IsType<Correspondence>(((CreatedAtActionResult)result.Result!).Value);
+        var stored = await db.Correspondences.SingleAsync();
+        Assert.Equal("manual", stored.Provider);
+    }
+}

JobTrackerApi.Tests/GmailControllerTests.cs

@@ -288,6 +288,7 @@ public sealed class GmailControllerTests
 
         var storedMessages = await db.Correspondences.Where(message => message.JobApplicationId == job.Id).ToListAsync();
         Assert.Single(storedMessages);
+        Assert.Equal("gmail", storedMessages[0].Provider);
         gmail.Verify(service => service.GetMessageAsync("user-1", "msg-1", It.IsAny<CancellationToken>()), Times.Once);
     }
 

JobTrackerApi/Controllers/CorrespondenceController.cs

@@ -159,6 +159,7 @@ namespace JobTrackerApi.Controllers
                 ExternalTo = string.IsNullOrWhiteSpace(request.ExternalTo) ? null : request.ExternalTo.Trim(),
                 ExternalLabelsJson = string.IsNullOrWhiteSpace(request.ExternalLabelsJson) ? null : request.ExternalLabelsJson.Trim(),
                 AttachmentMetadataJson = string.IsNullOrWhiteSpace(request.AttachmentMetadataJson) ? null : request.AttachmentMetadataJson.Trim(),
+                Provider = "manual",
                 Content = request.Content,
                 Date = request.Date ?? DateTime.Now,
             };

JobTrackerApi/Controllers/GmailController.cs

@@ -977,6 +977,7 @@ public sealed class GmailController : ControllerBase
                 GmailAttachmentId = attachment.ExternalAttachmentId,
                 Inline = attachment.Inline,
             })),
+            Provider = "gmail",
             Content = string.IsNullOrWhiteSpace(detail.BodyText) ? detail.Snippet : detail.BodyText,
             Date = messageDate,
         };

JobTrackerApi/Services/StartupInitializationExtensions.cs

@@ -529,6 +529,12 @@ public static class StartupInitializationExtensions
                 EnsureColumn(conn, "Correspondences", "Direction", "ALTER TABLE Correspondences ADD COLUMN Direction TEXT NULL;");
                 EnsureColumn(conn, "Correspondences", "ExternalLabelsJson", "ALTER TABLE Correspondences ADD COLUMN ExternalLabelsJson TEXT NULL;");
                 EnsureColumn(conn, "Correspondences", "AttachmentMetadataJson", "ALTER TABLE Correspondences ADD COLUMN AttachmentMetadataJson TEXT NULL;");
+                EnsureColumn(conn, "Correspondences", "Provider", "ALTER TABLE Correspondences ADD COLUMN Provider TEXT NULL;");
+                // Backfill: historically the only import source was Gmail (rows with an
+                // ExternalThreadId); everything else was hand-entered. Idempotent — only touches
+                // rows the app hasn't tagged yet.
+                Exec(conn, "UPDATE Correspondences SET Provider = 'gmail' WHERE Provider IS NULL AND ExternalThreadId IS NOT NULL;");
+                Exec(conn, "UPDATE Correspondences SET Provider = 'manual' WHERE Provider IS NULL;");
                 EnsureColumn(conn, "Attachments", "Purpose", "ALTER TABLE Attachments ADD COLUMN Purpose TEXT NULL;");
                 EnsureColumn(conn, "Attachments", "UseForAi", "ALTER TABLE Attachments ADD COLUMN UseForAi INTEGER NOT NULL DEFAULT 1;");
 
@@ -682,6 +688,17 @@ public static class StartupInitializationExtensions
                     EnsureMySqlColumn(conn, "Correspondences", "Direction", "ALTER TABLE `Correspondences` ADD COLUMN `Direction` varchar(100) NULL;");
                     EnsureMySqlColumn(conn, "Correspondences", "ExternalLabelsJson", "ALTER TABLE `Correspondences` ADD COLUMN `ExternalLabelsJson` longtext NULL;");
                     EnsureMySqlColumn(conn, "Correspondences", "AttachmentMetadataJson", "ALTER TABLE `Correspondences` ADD COLUMN `AttachmentMetadataJson` longtext NULL;");
+                    EnsureMySqlColumn(conn, "Correspondences", "Provider", "ALTER TABLE `Correspondences` ADD COLUMN `Provider` varchar(50) NULL;");
+                    using (var backfillGmail = conn.CreateCommand())
+                    {
+                        backfillGmail.CommandText = "UPDATE `Correspondences` SET `Provider` = 'gmail' WHERE `Provider` IS NULL AND `ExternalThreadId` IS NOT NULL;";
+                        backfillGmail.ExecuteNonQuery();
+                    }
+                    using (var backfillManual = conn.CreateCommand())
+                    {
+                        backfillManual.CommandText = "UPDATE `Correspondences` SET `Provider` = 'manual' WHERE `Provider` IS NULL;";
+                        backfillManual.ExecuteNonQuery();
+                    }
                     EnsureMySqlColumn(conn, "Attachments", "Purpose", "ALTER TABLE `Attachments` ADD COLUMN `Purpose` varchar(100) NULL;");
                     EnsureMySqlColumn(conn, "Attachments", "UseForAi", "ALTER TABLE `Attachments` ADD COLUMN `UseForAi` tinyint(1) NOT NULL DEFAULT 1;");
                     EnsureMySqlColumn(conn, "AspNetUsers", "ProfileCvText", "ALTER TABLE `AspNetUsers` ADD COLUMN `ProfileCvText` longtext NULL;");

Models/Correspondence.cs

@@ -21,6 +21,10 @@ namespace JobTrackerApi.Models
         public string? ExternalTo { get; set; }
         public string? ExternalLabelsJson { get; set; }
         public string? AttachmentMetadataJson { get; set; }
+        // Provider discriminator: "gmail" | "microsoft" | "imap" | "manual". Set at the write
+        // site (import controller or the manual-entry endpoint), not inferred from other fields,
+        // so it stays correct even for hand-entered rows that happen to carry external-looking data.
+        public string? Provider { get; set; }
         public string Content { get; set; } = "";
         public DateTime Date { get; set; } = DateTime.Now;