Improve backend DB concurrency and async safety

Tune SQLite and remove blocking calls from async/scan paths:

- Enable WAL + synchronous=NORMAL + busy_timeout + foreign_keys on each
  pooled connection, so the five scanners, web server, and retention no
  longer contend on the default rollback journal / FULL fsync.
- Run DB work in axum handlers via spawn_blocking (db::run_blocking) so
  synchronous rusqlite calls no longer block tokio worker threads.
- Deliver notifications on a dedicated task fed by a bounded channel; the
  blocking Pushover HTTP call runs in spawn_blocking, so a slow or
  unreachable Pushover can never stall device discovery.
- Make get_db_connection() return Result instead of panicking, so pool
  exhaustion surfaces as a 500 rather than crashing the process.

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

.gitignore

@@ -2,5 +2,7 @@
 /backend/target
 /result
 oott.db
+oott.db-wal
+oott.db-shm
 oott.toml
 /.claude