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.
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