fix(jobs): derive attachment checklist flags from actual Attachments

Backlog item 4 (Wave 3, first sub-item). HasResume/HasCoverLetter/HasPortfolio/
HasOtherAttachment were manually-editable checkboxes in EditJobDialog,
completely independent of whether a file was actually attached -- classic
drift: mark 'resume ready' by hand, later delete the resume attachment, flag
stays stuck true forever. User confirmed (asked directly, since removing the
manual-override capability is a product decision, not purely technical):
make them fully computed from Attachments, no manual override.

- AttachmentsController.RecomputeAttachmentFlagsAsync: the single place these
  four fields get written now, called after every attachment mutation
  (upload, delete, Purpose change) that could affect them. Deliberately kept
  as persisted columns (not [NotMapped] computed properties reading the
  Attachments navigation collection) -- ~15 query sites build JobApplication
  DTOs without .Include(Attachments), so a live-computed property would
  silently return false everywhere instead of throwing, the worst kind of
  bug. Recomputing at the one write funnel avoids touching any read path.
- Removed HasResume/etc from CreateJobApplicationRequest/
  UpdateJobApplicationRequest -- no longer client-settable.
- EditJobDialog: removed the manual checkboxes, kept the (now genuinely
  accurate) read-only status chips.
- AddJobModal: stopped sending has*-flags at job-creation time; the
  follow-up attachment upload call now sets them correctly via the same
  recompute path.

Caught a real bug while testing this: the Purpose-change path recomputed
before saving the Purpose change, so a fresh query missed the pending edit
and the flags never updated. Fixed by committing the mutation before
recomputing.

3 new backend tests (purpose-change sets flag, delete clears flag,
non-primary purpose counts as "other"). 172/172 backend, 25/25 frontend
suites (57 tests) green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Models/JobApplication.cs

@@ -24,7 +24,9 @@ public class JobApplication
     public DateTime? FeedbackRequestedAt { get; set; }
     public string? RecruiterMessageDraft { get; set; }
 
-    // Attachment checklist
+    // Attachment checklist. Derived from Attachment rows, not directly settable by API
+    // consumers -- see AttachmentsController.RecomputeAttachmentFlagsAsync, the single place
+    // these are written, so they can't drift from what's actually attached.
     public bool HasResume { get; set; } = false;
     public bool HasCoverLetter { get; set; } = false;
     public bool HasPortfolio { get; set; } = false;