Add SQLite-backed registry with JSON migration support

[kdush]

Summary

Add SQLite-backed registry as the default storage backend, with automatic JSON migration support.

Changes

• add DbRegistry class with SQLite backend, WAL mode, and JSON migration
• add storage_backend config option (sqlite | json)
• update CLI commands to use get_registry() factory
• document storage backend options and migration behavior
• add tests for SQLite registry, migration, and backend selection

Testing

• 25 new tests passed for SQLite backend and migration
• all existing tests pass
• end-to-end verified: init, add, query, status, list
• confirmed SQLite creates hashes.db, hashes.db-wal, hashes.db-shm

Backward Compatibility

• storage_backend: json still works for existing setups
• automatic migration from hashes.json to hashes.db when switching to SQLite
• hashes.json preserved after migration for safety