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>

jwt/database.go

@@ -0,0 +1,48 @@
+package jwt
+
+// DatabaseType specifies the database system and version being used.
+type DatabaseType struct {
+	Type    string // Database type: "postgres", "mysql", "sqlite", "mariadb"
+	Version string // Version string, e.g., "15.3", "8.0.32", "3.42.0"
+}
+
+// Predefined database type constants for easy configuration and validation.
+const (
+	DatabasePostgreSQL = "postgres"
+	DatabaseMySQL      = "mysql"
+	DatabaseSQLite     = "sqlite"
+	DatabaseMariaDB    = "mariadb"
+)
+
+// TableConfig configures the JWT blacklist table.
+type TableConfig struct {
+	// TableName is the name of the blacklist table.
+	// Default: "jwtblacklist"
+	TableName string
+
+	// AutoCreate determines whether to automatically create the table if it doesn't exist.
+	// Default: true
+	AutoCreate bool
+
+	// EnableAutoCleanup configures database-native automatic cleanup of expired tokens.
+	// For PostgreSQL: Creates a cleanup function (requires external scheduler or pg_cron)
+	// For MySQL/MariaDB: Creates a database event
+	// For SQLite: No automatic cleanup (manual only)
+	// Default: true
+	EnableAutoCleanup bool
+
+	// CleanupInterval specifies how often automatic cleanup should run (in hours).
+	// Only used if EnableAutoCleanup is true.
+	// Default: 24 (daily cleanup)
+	CleanupInterval int
+}
+
+// DefaultTableConfig returns a TableConfig with sensible defaults.
+func DefaultTableConfig() TableConfig {
+	return TableConfig{
+		TableName:         "jwtblacklist",
+		AutoCreate:        true,
+		EnableAutoCleanup: true,
+		CleanupInterval:   24,
+	}
+}