- Store interface with 9 methods covering all persistence operations
- SQLiteStore implements all 9 methods with exact SQL from current handlers
- NewSQLiteStore sets MaxOpenConns(1) and PRAGMA foreign_keys = ON
- UpsertEvent uses ON CONFLICT DO UPDATE with acknowledged_at reset to NULL
- AssignTag uses INSERT OR REPLACE for tag_assignments table
- golang-migrate v4.19.1 dependency added to go.mod
