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/EmailDraftsControllerTests.cs

@@ -40,6 +40,7 @@ public sealed class EmailDraftsControllerTests
         Assert.Equal("gmail", created.Provider);
         Assert.Equal(1, created.Revision);
         Assert.Empty(created.To);
+        Assert.True(Guid.TryParse(created.ClientRequestId, out _));
 
         Assert.IsType<BadRequestObjectResult>((await controller.Create(
             new EmailDraftsController.CreateDraftRequest(fixture.UserOneJobId, "unknown", "", "", "", null), default)).Result);
@@ -63,6 +64,9 @@ public sealed class EmailDraftsControllerTests
         var ownList = Assert.IsAssignableFrom<IReadOnlyList<EmailDraftsController.DraftDto>>(
             Assert.IsType<OkObjectResult>((await controller.List(fixture.UserOneJobId, default)).Result).Value);
         Assert.Equal(fixture.UserOneDraftId, Assert.Single(ownList).Id);
+        var allOwnDrafts = Assert.IsAssignableFrom<IReadOnlyList<EmailDraftsController.DraftDto>>(
+            Assert.IsType<OkObjectResult>((await controller.List(null, default)).Result).Value);
+        Assert.Equal(fixture.UserOneDraftId, Assert.Single(allOwnDrafts).Id);
     }
 
     [Fact]
@@ -72,11 +76,13 @@ public sealed class EmailDraftsControllerTests
         await using (var userOneDb = fixture.Context("user-1"))
         {
             var userOne = Controller(userOneDb, "user-1");
+            var originalRequestId = (await userOneDb.EmailDrafts.AsNoTracking().SingleAsync()).ClientRequestId;
             var updatedResult = await userOne.Update(fixture.UserOneDraftId,
                 new EmailDraftsController.UpdateDraftRequest(1, "new@example.test", "Updated", "Updated body"), default);
             var updated = Assert.IsType<EmailDraftsController.DraftDto>(Assert.IsType<OkObjectResult>(updatedResult.Result).Value);
             Assert.Equal(2, updated.Revision);
             Assert.Equal("Updated body", updated.BodyText);
+            Assert.Equal(originalRequestId, updated.ClientRequestId);
 
             Assert.IsType<ConflictObjectResult>((await userOne.Update(fixture.UserOneDraftId,
                 new EmailDraftsController.UpdateDraftRequest(1, "stale@example.test", "Stale", "Stale body"), default)).Result);
@@ -190,6 +196,7 @@ public sealed class EmailDraftsControllerTests
             To = recipient,
             Subject = "Synthetic",
             BodyText = "Synthetic private draft.",
+            ClientRequestId = Guid.NewGuid().ToString("D"),
             CreatedAtUtc = DateTime.UtcNow,
             UpdatedAtUtc = DateTime.UtcNow,
         };