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/tokengen_test.go

@@ -7,14 +7,16 @@ import (
 )
 
 func newTestGenerator(t *testing.T) *TokenGenerator {
-	gen, err := CreateGenerator(
-		15,
-		60,
-		5,
-		"example.com",
-		"supersecret",
-		nil,
-	)
+	gen, err := CreateGenerator(GeneratorConfig{
+		AccessExpireAfter:  15,
+		RefreshExpireAfter: 60,
+		FreshExpireAfter:   5,
+		TrustedHost:        "example.com",
+		SecretKey:          "supersecret",
+		DB:                 nil,
+		DBType:             DatabaseType{Type: DatabasePostgreSQL, Version: "15"},
+		TableConfig:        DefaultTableConfig(),
+	})
 	require.NoError(t, err)
 	return gen
 }