feat(ai): AI Workspace per job application — modules + append-only history

Phase 5 backend. A unified AI Workspace for each application, orchestrating the
five suggestion modules through the existing ISummarizerService provider
abstraction and storing every generation as append-only history (AiInteraction)
so outputs can be reused, compared, and deleted — distinct from the existing
AiWorkspaceNote cache (one row, overwritten).

Modules (all suggestion-only, "never invent facts" guardrail, never mutate the
profile/variant/application): job-analysis, career-match, cover-letter (6 modes),
interview, application-review. Each builds a prompt from the job + master profile
text and returns markdown.

- Models/AiInteraction.cs + migration AddAiInteractions (verified on container)
- Services/AiWorkspaceService.cs (prompts, history, delete)
- Controllers/AiWorkspaceController.cs (/api/jobapplications/{id}/ai:
  generate, history, delete, modules+provider)
- 7 tests (store, history filter/order, delete, mode normalization, unknown
  module, empty output, tenant scoping); 306 backend green

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

Data/JobTrackerContext.cs

@@ -44,6 +44,7 @@ namespace JobTrackerApi.Data
     public DbSet<AiWorkspaceNote> AiWorkspaceNotes => Set<AiWorkspaceNote>();
     public DbSet<CvVariant> CvVariants => Set<CvVariant>();
     public DbSet<CvVariantVersion> CvVariantVersions => Set<CvVariantVersion>();
+    public DbSet<AiInteraction> AiInteractions => Set<AiInteraction>();
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -317,6 +318,19 @@ namespace JobTrackerApi.Data
             .WithMany()
             .HasForeignKey(x => x.CvVariantId)
             .OnDelete(DeleteBehavior.Cascade);
+
+        // Phase 5: append-only AI interaction history per job application. Same deny-on-null tenant
+        // filter; indexed for the per-job history read; cascades with the application.
+        // docs/architecture/ai-career-assistant.md.
+        modelBuilder.Entity<AiInteraction>()
+            .HasQueryFilter(x => CurrentUserId != null && x.OwnerUserId == CurrentUserId);
+        modelBuilder.Entity<AiInteraction>()
+            .HasIndex(x => new { x.OwnerUserId, x.JobApplicationId, x.Module, x.CreatedAtUtc });
+        modelBuilder.Entity<AiInteraction>()
+            .HasOne(x => x.JobApplication)
+            .WithMany()
+            .HasForeignKey(x => x.JobApplicationId)
+            .OnDelete(DeleteBehavior.Cascade);
     }
 
     // Common config for CareerProfile's relational children. The 1:many FK + cascade delete is