fix(email): preserve draft send identity

Persist and export each draft's idempotency UUID so refresh and edits cannot create a fresh delivery identity. Add reversible provider-specific migration SQL.

JobTrackerApi.Tests/EmailDraftPersistenceTests.cs

@@ -64,6 +64,7 @@ public sealed class EmailDraftPersistenceTests
         Subject = "Synthetic draft",
         BodyText = "Synthetic private draft body.",
         ThreadId = "thread-1",
+        ClientRequestId = Guid.NewGuid().ToString("D"),
         CreatedAtUtc = DateTime.UtcNow,
         UpdatedAtUtc = DateTime.UtcNow,
     };