{
  "title": "SQLite cache for list performance",
  "body": "Add an optional SQLite cache in `.nbd/cache.db` to accelerate `nbd list` and `nbd ready` for large ticket stores.\n\n## Motivation\n\n`list_tickets` currently does O(n) file reads on every call. For stores with hundreds of tickets this is measurably slow. A SQLite cache avoids re-reading unchanged files by comparing file modification times (mtimes).\n\n## Approach\n\n### Crate dependency\nAdd `sqlx` with the `sqlite` and `runtime-async-std` features to `Cargo.toml`.\n\n### store.rs additions\nNew async function `open_cache(root: &Path) -> Result<sqlx::SqlitePool>`:\n- Opens (or creates) `.nbd/cache.db`.\n- Runs a migration: `CREATE TABLE IF NOT EXISTS tickets (id TEXT PRIMARY KEY, json TEXT NOT NULL, mtime INTEGER NOT NULL)`.\n\nNew function `list_tickets_cached(root: &Path) -> Result<Vec<Ticket>>`:\n1. Open cache.\n2. Read directory listing to get file names and mtimes.\n3. For each file: if the DB has a row with matching mtime, use cached JSON; otherwise read file, parse, insert/update row.\n4. Delete DB rows for IDs no longer on disk.\n5. Return deserialized tickets sorted by priority desc.\n\nKeep existing `list_tickets` as the non-cached fallback. Consider making `cmd_list` and `cmd_ready` use `list_tickets_cached` when available.\n\n### Migration strategy\n- The cache is always optional. If `cache.db` can't be opened, fall back to `list_tickets` (log a warning to stderr).\n- The cache is never the source of truth — the JSON files are. The cache is always reconstructable by deleting `.nbd/cache.db`.\n\n## Decision point\nDecide whether to enable the cache unconditionally or gate it behind a flag (`--cache` / `NBD_CACHE=1`). Recommendation: enable by default once the feature is stable.\n\n## Tests\n- Unit test: cache hit returns same data as direct file read.\n- Unit test: cache miss (mtime changed) re-reads the file.\n- Unit test: deleted ticket is evicted from cache.\n- Performance test (optional): benchmark 1000-ticket list with and without cache.\n\n## Files touched\n- `Cargo.toml` — add `sqlx`\n- `src/store.rs` — `open_cache`, `list_tickets_cached`\n- `src/tests.rs` — cache unit tests\n- `docs/ARCHITECTURE.md` — document the cache layer",
  "priority": 3,
  "status": "todo",
  "dependencies": [],
  "ticket_type": "feature"
}