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/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,
         };