feat: persist interview prep instead of regenerating on every open

Interview prep re-ran its AI call every time the tab opened -- flagged
in the product teardown as work evaporating on every re-open (cost,
latency, and non-determinism for no reason). GetInterviewPrep now
persists one note per job application and reuses it on subsequent
reads, only regenerating when the selected attachment context changes
or a refresh is explicitly requested.

- InterviewPrepNote: one row per (owner, job), keyed additionally by
  an attachment-selection fingerprint so picking different attachments
  correctly triggers a fresh brief without needing an explicit flag.
- GetInterviewPrep gained a `refresh` query param; the frontend adds a
  small "Regenerate" button as the explicit escape hatch for when the
  underlying job/notes have changed since the note was written.
- Both SQLite (dev) and MySQL/MariaDB (prod) reconciler dialects.
- 3 new tests: reuse across calls, refresh regenerates, attachment
  context change regenerates. Verified against the real dev DB.

JobTrackerApi/Controllers/JobApplicationsController.cs

@@ -2398,13 +2398,31 @@ Candidate master CV:
         }
 
         [HttpGet("{id:int}/interview-prep")]
-        public async Task<ActionResult<InterviewPrepDto>> GetInterviewPrep([FromRoute] int id, [FromQuery] string? attachmentIds, CancellationToken cancellationToken)
+        public async Task<ActionResult<InterviewPrepDto>> GetInterviewPrep([FromRoute] int id, [FromQuery] string? attachmentIds, [FromQuery] bool refresh, CancellationToken cancellationToken)
         {
             var job = await _db.JobApplications
                 .Include(j => j.Company)
                 .FirstOrDefaultAsync(j => j.Id == id, cancellationToken);
             if (job is null) return NotFound();
 
+            var userId = CurrentUserId;
+            var attachmentSignature = NormalizeAttachmentIdsSignature(attachmentIds);
+
+            if (!refresh && userId is not null)
+            {
+                var existing = await _db.InterviewPrepNotes.FirstOrDefaultAsync(
+                    x => x.OwnerUserId == userId && x.JobApplicationId == id && x.AttachmentContextSignature == attachmentSignature,
+                    cancellationToken);
+                if (existing is not null)
+                {
+                    return Ok(new InterviewPrepDto(
+                        existing.Summary,
+                        JsonSerializer.Deserialize<List<string>>(existing.TalkingPointsJson) ?? new List<string>(),
+                        JsonSerializer.Deserialize<List<string>>(existing.LikelyQuestionsJson) ?? new List<string>(),
+                        JsonSerializer.Deserialize<List<string>>(existing.WeakSpotsJson) ?? new List<string>()));
+                }
+            }
+
             var attachmentContext = await BuildAttachmentContextAsync(id, cancellationToken, attachmentIds);
             var context = string.Join("\n\n", new[] { job.JobTitle, job.Company?.Name, job.Description, job.TranslatedDescription, job.Notes, job.ShortSummary, attachmentContext?.Context }
                 .Where(x => !string.IsNullOrWhiteSpace(x)));
@@ -2423,9 +2441,34 @@ Candidate master CV:
                 180,
                 70) ?? "Prepare concise, outcome-focused stories that match the core role requirements.";
 
+            if (userId is not null)
+            {
+                var note = await _db.InterviewPrepNotes.FirstOrDefaultAsync(x => x.OwnerUserId == userId && x.JobApplicationId == id, cancellationToken);
+                if (note is null)
+                {
+                    note = new InterviewPrepNote { OwnerUserId = userId, JobApplicationId = id };
+                    _db.InterviewPrepNotes.Add(note);
+                }
+                note.AttachmentContextSignature = attachmentSignature;
+                note.Summary = summary;
+                note.TalkingPointsJson = JsonSerializer.Serialize(talkingPoints);
+                note.LikelyQuestionsJson = JsonSerializer.Serialize(likelyQuestions);
+                note.WeakSpotsJson = JsonSerializer.Serialize(weakSpots);
+                note.GeneratedAtUtc = DateTimeOffset.UtcNow;
+                await _db.SaveChangesAsync(cancellationToken);
+            }
+
             return Ok(new InterviewPrepDto(summary, talkingPoints, likelyQuestions, weakSpots));
         }
 
+        private static string NormalizeAttachmentIdsSignature(string? attachmentIds)
+        {
+            if (string.IsNullOrWhiteSpace(attachmentIds)) return string.Empty;
+            var ids = attachmentIds.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+                .OrderBy(x => x, StringComparer.Ordinal);
+            return string.Join(",", ids);
+        }
+
         [HttpGet("{id:int}/readiness")]
         public async Task<ActionResult<ReadinessDto>> GetReadiness([FromRoute] int id, CancellationToken cancellationToken)
         {