feat: persist candidate fit and focus plan, stop re-running on every open

Extends the interview-prep persistence pattern (previous commit) to
the other two AI-generated per-job outputs that were re-running their
full AI call chain on every tab open: candidate-fit (4 AI calls) and
focus-plan (4 AI calls). Across all three tabs that's 9 AI calls fired
every single time a user revisits a job's AI workspace tabs.

Generalized into AiWorkspaceNote (OwnerUserId, JobApplicationId,
NoteType, ResultJson) rather than duplicating InterviewPrepNote's
per-field-column shape: CandidateFitDto and FocusPlanDto are irregular
and nested (up to 13 fields including a nested guidance object),
where per-field columns would be unreasonable. One table, keyed by
note type, serving both.

Same rules as interview prep: reuse across calls, regenerate when the
attachment selection changes, regenerate on explicit refresh. Frontend
gets the same "Regenerate" button on both tabs.

3 new tests (persist+reuse for both, refresh for candidate-fit).
Verified against the real dev DB.

Data/JobTrackerContext.cs

@@ -31,6 +31,7 @@ namespace JobTrackerApi.Data
     public DbSet<CareerProfile> CareerProfiles => Set<CareerProfile>();
     public DbSet<CareerProfileVersion> CareerProfileVersions => Set<CareerProfileVersion>();
     public DbSet<InterviewPrepNote> InterviewPrepNotes => Set<InterviewPrepNote>();
+    public DbSet<AiWorkspaceNote> AiWorkspaceNotes => Set<AiWorkspaceNote>();
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -179,6 +180,22 @@ namespace JobTrackerApi.Data
             .WithMany()
             .HasForeignKey(x => x.JobApplicationId)
             .OnDelete(DeleteBehavior.Cascade);
+
+        // Generic AI workspace note persistence (candidate fit, focus plan) -- same rationale as
+        // InterviewPrepNote above, generalized because these DTOs are irregular/nested enough that
+        // per-field columns would be unreasonable.
+        modelBuilder.Entity<AiWorkspaceNote>()
+            .HasQueryFilter(x => CurrentUserId != null && x.OwnerUserId == CurrentUserId);
+
+        modelBuilder.Entity<AiWorkspaceNote>()
+            .HasIndex(x => new { x.OwnerUserId, x.JobApplicationId, x.NoteType })
+            .IsUnique();
+
+        modelBuilder.Entity<AiWorkspaceNote>()
+            .HasOne(x => x.JobApplication)
+            .WithMany()
+            .HasForeignKey(x => x.JobApplicationId)
+            .OnDelete(DeleteBehavior.Cascade);
     }
 }
 }