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);
+    }
+}