feat(email): add explicit send API

Require tenant-owned jobs, explicit confirmation, canonical request IDs, and rate limiting before provider delivery. Persist sent correspondence with a content-free idempotency ledger, and never retry uncertain outcomes automatically.

JobTrackerApi/Services/EmailSendAttemptStore.cs

@@ -26,7 +26,7 @@ public sealed class EmailSendAttemptStore(JobTrackerContext db, TimeProvider tim
         if (!await db.JobApplications.AnyAsync(job => job.Id == request.JobApplicationId, cancellationToken))
             throw new InvalidOperationException("The job application does not exist in the current owner scope.");
 
-        var existing = await db.EmailSendAttempts.FirstOrDefaultAsync(item => item.ClientRequestId == request.ClientRequestId, cancellationToken);
+        var existing = await db.EmailSendAttempts.AsNoTracking().FirstOrDefaultAsync(item => item.ClientRequestId == request.ClientRequestId, cancellationToken);
         if (existing is not null) return Existing(existing, request.PayloadHash);
 
         var attempt = new EmailSendAttempt
@@ -48,7 +48,7 @@ public sealed class EmailSendAttemptStore(JobTrackerContext db, TimeProvider tim
         catch (DbUpdateException)
         {
             db.Entry(attempt).State = EntityState.Detached;
-            existing = await db.EmailSendAttempts.FirstOrDefaultAsync(item => item.ClientRequestId == request.ClientRequestId, cancellationToken);
+            existing = await db.EmailSendAttempts.AsNoTracking().FirstOrDefaultAsync(item => item.ClientRequestId == request.ClientRequestId, cancellationToken);
             if (existing is not null) return Existing(existing, request.PayloadHash);
             throw;
         }