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>

JobTrackerApi/Controllers/JobApplicationsController.cs

@@ -1376,11 +1376,7 @@ Canonical profile:
             string? CoverLetterText,
             string? JobUrl,
             DateTime? DateApplied,
-            DateTime? FeedbackRequestedAt,
-            bool? HasResume,
-            bool? HasCoverLetter,
-            bool? HasPortfolio,
-            bool? HasOtherAttachment
+            DateTime? FeedbackRequestedAt
         );
 
         private static (decimal? Min, decimal? Max, string? Currency, string? Period) NormalizeSalary(
@@ -1422,10 +1418,9 @@ Canonical profile:
                 NextAction = string.IsNullOrWhiteSpace(request.NextAction) ? null : request.NextAction.Trim(),
                 FollowUpAt = request.FollowUpAt,
                 FeedbackRequestedAt = request.FeedbackRequestedAt,
-                HasResume = request.HasResume ?? false,
-                HasCoverLetter = request.HasCoverLetter ?? false,
-                HasPortfolio = request.HasPortfolio ?? false,
-                HasOtherAttachment = request.HasOtherAttachment ?? false,
+                // HasResume/HasCoverLetter/HasPortfolio/HasOtherAttachment are derived from
+                // Attachment rows (see AttachmentsController.RecomputeAttachmentFlagsAsync), not
+                // settable here -- they start false and get set correctly once files are uploaded.
                 Notes = string.IsNullOrWhiteSpace(request.Notes) ? null : request.Notes,
                 Description = string.IsNullOrWhiteSpace(request.Description) ? null : request.Description,
                 TranslatedDescription = string.IsNullOrWhiteSpace(request.TranslatedDescription) ? null : request.TranslatedDescription,
@@ -1486,10 +1481,6 @@ Canonical profile:
             string? SalaryPeriod,
             string? NextAction,
             DateTime? FollowUpAt,
-            bool? HasResume,
-            bool? HasCoverLetter,
-            bool? HasPortfolio,
-            bool? HasOtherAttachment,
             string? Notes,
             string? Description,
             string? TranslatedDescription,
@@ -1529,10 +1520,8 @@ Canonical profile:
             job.NextAction = string.IsNullOrWhiteSpace(request.NextAction) ? null : request.NextAction.Trim();
             job.FollowUpAt = request.FollowUpAt;
             job.FeedbackRequestedAt = request.FeedbackRequestedAt;
-            if (request.HasResume is not null) job.HasResume = request.HasResume.Value;
-            if (request.HasCoverLetter is not null) job.HasCoverLetter = request.HasCoverLetter.Value;
-            if (request.HasPortfolio is not null) job.HasPortfolio = request.HasPortfolio.Value;
-            if (request.HasOtherAttachment is not null) job.HasOtherAttachment = request.HasOtherAttachment.Value;
+            // HasResume/HasCoverLetter/HasPortfolio/HasOtherAttachment are derived from
+            // Attachment rows, not settable here -- see AttachmentsController.RecomputeAttachmentFlagsAsync.
             job.Notes = request.Notes;
             job.Description = request.Description;
             job.TranslatedDescription = request.TranslatedDescription;