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

migrations/2026-05-27-000000_add_insight_generation_jobs/up.sql

@@ -0,0 +1,25 @@
+-- Track async insight generation jobs so the client can poll for
+-- completion after the server returns 202 Accepted. The UNIQUE
+-- constraint on (library_id, file_path, generation_type) ensures
+-- idempotent inserts: if a running job already exists, the caller
+-- should return that job_id instead of creating a duplicate.
+CREATE TABLE insight_generation_jobs (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    library_id INTEGER NOT NULL DEFAULT 1,
+    file_path TEXT NOT NULL,
+    generation_type TEXT NOT NULL,
+    status TEXT NOT NULL DEFAULT 'running',
+    started_at INTEGER NOT NULL,
+    completed_at INTEGER,
+    result_insight_id INTEGER,
+    error_message TEXT,
+    UNIQUE(library_id, file_path, generation_type)
+);
+
+-- For the status endpoint: fast lookup by (library_id, file_path)
+CREATE INDEX idx_insight_gen_jobs_file
+    ON insight_generation_jobs(library_id, file_path);
+
+-- For startup cleanup (future): prune old completed/failed jobs
+CREATE INDEX idx_insight_gen_jobs_status_cleanup
+    ON insight_generation_jobs(status, started_at);