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>

JobTrackerApi/Program.cs

@@ -40,6 +40,7 @@ builder.Services.AddSingleton<IThemedCvRenderer, ThemedCvRenderer>();
 builder.Services.AddSingleton<ICvPdfExporter, PlaywrightCvPdfExporter>();
 builder.Services.AddScoped<ICareerProfileService, CareerProfileService>();
 builder.Services.AddScoped<ICvVariantService, CvVariantService>();
+builder.Services.AddScoped<IAiWorkspaceService, AiWorkspaceService>();
 
 builder.Services.AddSingleton<AppPaths>();
 builder.Services.AddSingleton<IStartupReadiness, StartupReadiness>();