Consolidate SQLite and Postgres store implementations into a single
commonStore with database-specific SQL queries passed via configuration.
This eliminates ~100 lines of duplicate code while maintaining full
functionality for both backends.
Key changes:
- Move all store methods to commonStore in store.go
- Remove sqliteStore and postgresStore wrapper structs
- Refactor SQLite to use QueryRow() pattern like Postgres
- Pass database-specific queries via storeQueries struct
- Make store types unexported, only expose Store interface
All tests pass for both SQLite and PostgreSQL backends.
- Add SetSubscriptionUpdatedAt to Store interface, remove DB() accessor
- Rename store files to store_sqlite.go and store_postgres.go
- Use camelCase for test function names
- Add tests for upsert field updates and multi-user removal
- Use transaction in setupNewPostgresDB
- Use lowercase "excluded." in PostgreSQL upsert query
