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.

Data/JobTrackerContext.cs

@@ -30,6 +30,7 @@ namespace JobTrackerApi.Data
     public DbSet<TailoredCvDraft> TailoredCvDrafts => Set<TailoredCvDraft>();
     public DbSet<CareerProfile> CareerProfiles => Set<CareerProfile>();
     public DbSet<CareerProfileVersion> CareerProfileVersions => Set<CareerProfileVersion>();
+    public DbSet<InterviewPrepNote> InterviewPrepNotes => Set<InterviewPrepNote>();
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -164,6 +165,20 @@ namespace JobTrackerApi.Data
             .WithMany()
             .HasForeignKey(x => x.CareerProfileId)
             .OnDelete(DeleteBehavior.Cascade);
+
+        // Interview prep persistence (career-workspace-implementation-roadmap.md Phase F5).
+        modelBuilder.Entity<InterviewPrepNote>()
+            .HasQueryFilter(x => CurrentUserId != null && x.OwnerUserId == CurrentUserId);
+
+        modelBuilder.Entity<InterviewPrepNote>()
+            .HasIndex(x => new { x.OwnerUserId, x.JobApplicationId })
+            .IsUnique();
+
+        modelBuilder.Entity<InterviewPrepNote>()
+            .HasOne(x => x.JobApplication)
+            .WithMany()
+            .HasForeignKey(x => x.JobApplicationId)
+            .OnDelete(DeleteBehavior.Cascade);
     }
 }
 }