Add shared test functions in store_test.go covering all Store interface
operations (users, tokens, access control, tiers, billing, stats, etc.)
with per-backend wrappers in store_sqlite_test.go and store_postgres_test.go
following the webpush test pattern.
Fix broken isUniqueConstraintError() which used incorrect interface
assertions instead of string matching for SQLite/PostgreSQL errors.
Extract database operations from Manager into a Store interface with
SQLite and PostgreSQL implementations using a shared commonStore.
Split SQLite migrations into store_sqlite_migrations.go, use shared
schema_version table for PostgreSQL, rename user_user/user_tier tables
to "user"/tier, and wire up database-url in CLI commands.
