fix(email): gate draft attempt rotation

Rotate persisted delivery identity only after the matching owner attempt is definitively failed. Keep stale, foreign, pending, sent, and uncertain drafts non-retryable.

JobTrackerApi/Controllers/EmailDraftsController.cs

@@ -39,6 +39,7 @@ public sealed class EmailDraftsController(
         string? ThreadId);
 
     public sealed record UpdateDraftRequest(long Revision, string? To, string? Subject, string? BodyText);
+    public sealed record NewAttemptRequest(long Revision);
 
     [HttpGet]
     public async Task<ActionResult<IReadOnlyList<DraftDto>>> List(
@@ -165,6 +166,40 @@ public sealed class EmailDraftsController(
             : NotFound();
     }
 
+    [HttpPost("{id:guid}/new-attempt")]
+    public async Task<ActionResult<DraftDto>> NewAttempt(Guid id, NewAttemptRequest request, CancellationToken cancellationToken)
+    {
+        var ownerUserId = GetOwnerUserId();
+        if (ownerUserId is null) return Unauthorized();
+        if (request.Revision <= 0) return BadRequest("A positive revision is required.");
+
+        var draft = await db.EmailDrafts.AsNoTracking()
+            .FirstOrDefaultAsync(item => item.Id == id && item.OwnerUserId == ownerUserId, cancellationToken);
+        if (draft is null) return NotFound();
+        if (draft.Revision != request.Revision)
+            return Conflict(new ProblemDetails { Title = "Draft revision conflict", Detail = "Reload the latest draft before creating a new attempt." });
+
+        var failedAttempt = await db.EmailSendAttempts.AsNoTracking().AnyAsync(attempt =>
+            attempt.OwnerUserId == ownerUserId &&
+            attempt.ClientRequestId == draft.ClientRequestId &&
+            attempt.Status == EmailSendStatuses.Failed,
+            cancellationToken);
+        if (!failedAttempt)
+            return Conflict(new ProblemDetails { Title = "A new attempt is not allowed", Detail = "Only a definitively failed delivery can receive a new attempt identity." });
+
+        var newClientRequestId = Guid.NewGuid().ToString("D");
+        var now = timeProvider.GetUtcNow().UtcDateTime;
+        var affected = await db.EmailDrafts
+            .Where(item => item.Id == id && item.OwnerUserId == ownerUserId && item.Revision == request.Revision && item.ClientRequestId == draft.ClientRequestId)
+            .ExecuteUpdateAsync(setters => setters
+                .SetProperty(item => item.ClientRequestId, newClientRequestId)
+                .SetProperty(item => item.Revision, item => item.Revision + 1)
+                .SetProperty(item => item.UpdatedAtUtc, now), cancellationToken);
+        if (affected != 1)
+            return Conflict(new ProblemDetails { Title = "Draft revision conflict", Detail = "Reload the latest draft before creating a new attempt." });
+        return await Get(id, cancellationToken);
+    }
+
     private string? GetOwnerUserId() =>
         User.FindFirstValue(ClaimTypes.NameIdentifier) ?? User.FindFirstValue("sub");