Refactor database interface to use *sql.DB directly

Simplified the database layer by removing custom interface wrappers
and using standard library *sql.DB and *sql.Tx types directly.

Changes:
- Removed DBConnection and DBTransaction interfaces from database.go
- Removed NewDBConnection() wrapper function
- Updated TokenGenerator to use *sql.DB instead of DBConnection
- Updated all validation and revocation methods to accept *sql.Tx
- Updated TableManager to work with *sql.DB directly
- Updated all tests to use db.Begin() instead of custom wrappers
- Fixed GeneratorConfig.DB field (was DBConn)
- Updated documentation in doc.go with correct API usage

Benefits:
- Simpler API with fewer abstractions
- Works directly with database/sql standard library
- Compatible with GORM (via gormDB.DB()) and Bun (share same *sql.DB)
- Easier to understand and maintain
- No unnecessary wrapper layers

Breaking changes:
- GeneratorConfig.DBConn renamed to GeneratorConfig.DB
- Removed NewDBConnection() function - pass *sql.DB directly
- ValidateAccess/ValidateRefresh now accept *sql.Tx instead of DBTransaction
- Token.Revoke/CheckNotRevoked now accept *sql.Tx instead of DBTransaction

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

hwsauth/db.go

@@ -0,0 +1,27 @@
+package hwsauth
+
+import (
+	"context"
+	"database/sql"
+)
+
+// DBTransaction represents a database transaction that can be committed or rolled back.
+// This interface can be implemented by standard library sql.Tx, or by ORM transactions
+// from libraries like bun, gorm, sqlx, etc.
+type DBTransaction interface {
+	Commit() error
+	Rollback() error
+}
+
+// DBConnection represents a database connection that can begin transactions.
+// This interface can be implemented by standard library sql.DB, or by ORM connections
+// from libraries like bun, gorm, sqlx, etc.
+type DBConnection interface {
+	BeginTx(ctx context.Context, opts *sql.TxOptions) (DBTransaction, error)
+}
+
+// Ensure *sql.Tx implements DBTransaction
+var _ DBTransaction = (*sql.Tx)(nil)
+
+// Ensure *sql.DB implements DBConnection
+var _ DBConnection = (*sql.DB)(nil)