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/validate_test.go

@@ -1,7 +1,6 @@
 package jwt
 
 import (
-	"context"
 	"database/sql"
 	"testing"
 
@@ -9,23 +8,6 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func newGeneratorWithMockDB(t *testing.T) (*TokenGenerator, sqlmock.Sqlmock, func()) {
-	db, mock, err := sqlmock.New()
-	require.NoError(t, err)
-
-	gen, err := CreateGenerator(
-		15,
-		60,
-		5,
-		"example.com",
-		"supersecret",
-		db,
-	)
-	require.NoError(t, err)
-
-	return gen, mock, func() { db.Close() }
-}
-
 func expectNotRevoked(mock sqlmock.Sqlmock, jti any) {
 	mock.ExpectBegin()
 	mock.ExpectQuery(`SELECT 1 FROM jwtblacklist`).
@@ -44,7 +26,7 @@ func TestValidateAccess_Success(t *testing.T) {
 	// We don't know the JTI beforehand; match any arg
 	expectNotRevoked(mock, sqlmock.AnyArg())
 
-	tx, err := gen.dbConn.BeginTx(context.Background(), nil)
+	tx, err := gen.db.Begin()
 	require.NoError(t, err)
 	defer tx.Rollback()
 
@@ -61,7 +43,10 @@ func TestValidateAccess_NoDB(t *testing.T) {
 	tokenStr, _, err := gen.NewAccess(42, true, false)
 	require.NoError(t, err)
 
-	token, err := gen.ValidateAccess(&sql.Tx{}, tokenStr)
+	// Use nil transaction for no-db case
+	var tx *sql.Tx = nil
+
+	token, err := gen.ValidateAccess(tx, tokenStr)
 	require.NoError(t, err)
 	require.Equal(t, 42, token.SUB)
 	require.Equal(t, "access", token.Scope)
@@ -76,7 +61,7 @@ func TestValidateRefresh_Success(t *testing.T) {
 
 	expectNotRevoked(mock, sqlmock.AnyArg())
 
-	tx, err := gen.dbConn.BeginTx(context.Background(), nil)
+	tx, err := gen.db.Begin()
 	require.NoError(t, err)
 	defer tx.Rollback()
 
@@ -93,7 +78,10 @@ func TestValidateRefresh_NoDB(t *testing.T) {
 	tokenStr, _, err := gen.NewRefresh(42, false)
 	require.NoError(t, err)
 
-	token, err := gen.ValidateRefresh(nil, tokenStr)
+	// Use nil transaction for no-db case
+	var tx *sql.Tx = nil
+
+	token, err := gen.ValidateRefresh(tx, tokenStr)
 	require.NoError(t, err)
 	require.Equal(t, 42, token.SUB)
 	require.Equal(t, "refresh", token.Scope)
@@ -102,7 +90,10 @@ func TestValidateRefresh_NoDB(t *testing.T) {
 func TestValidateAccess_EmptyToken(t *testing.T) {
 	gen := newTestGenerator(t)
 
-	_, err := gen.ValidateAccess(nil, "")
+	// Use nil transaction
+	var tx *sql.Tx = nil
+
+	_, err := gen.ValidateAccess(tx, "")
 	require.Error(t, err)
 }
 
@@ -113,6 +104,9 @@ func TestValidateRefresh_WrongScope(t *testing.T) {
 	tokenStr, _, err := gen.NewAccess(1, false, false)
 	require.NoError(t, err)
 
-	_, err = gen.ValidateRefresh(nil, tokenStr)
+	// Use nil transaction
+	var tx *sql.Tx = nil
+
+	_, err = gen.ValidateRefresh(tx, tokenStr)
 	require.Error(t, err)
 }