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/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);
     }