refactor: migrate data storage from JSON/map files to SQLite

Replace servers.json, certificates.json, and map file parsing with
SQLite (WAL mode) as single source of truth. HAProxy map files are
now generated from SQLite via sync_map_files().

Key changes:
- Add db.py with schema, connection management, and JSON migration
- Add DB_FILE config constant
- Delegate file_ops.py functions to db.py
- Refactor domains.py to use file_ops instead of direct list manipulation
- Fix subprocess.TimeoutExpired not caught (doesn't inherit TimeoutError)
- Add DB health check in health.py
- Init DB on startup in server.py and __main__.py
- Update all 359 tests to use SQLite-backed functions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -16,6 +16,9 @@ run/
 data/
 *.state
 *.lock
+*.db
+*.db-wal
+*.db-shm
 
 # Python
 __pycache__/