feat: async insight generation with SQLite job tracking

- Add insight_generation_jobs table migration and DAO
- Implement job lifecycle: create_or_get_active, complete, fail, cancel
- Refactor POST /insights/generate and /agentic to async spawn with timeout
- Add GET /insights/generation/status endpoint with job_id and file_path lookup
- Use String for enum fields in Diesel models to avoid private Bound type
- Add from_str() helpers on InsightJobStatus and InsightGenerationType
- Fix update_training_messages to return Result<usize, DbError>
- 7/7 DAO unit tests passing

src/ai/mod.rs

@@ -21,8 +21,9 @@ pub use daily_summary_job::{
 pub use handlers::{
     chat_history_handler, chat_rewind_handler, chat_stream_handler, chat_turn_handler,
     delete_insight_handler, export_training_data_handler, generate_agentic_insight_handler,
-    generate_insight_handler, get_all_insights_handler, get_available_models_handler,
-    get_insight_handler, get_openrouter_models_handler, rate_insight_handler,
+    generate_insight_handler, generation_status_handler, get_all_insights_handler,
+    get_available_models_handler, get_insight_handler, get_openrouter_models_handler,
+    rate_insight_handler,
 };
 pub use insight_generator::InsightGenerator;
 pub use llamacpp::LlamaCppClient;