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/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;");