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/reauthenticate.go

@@ -1,14 +1,13 @@
 package hwsauth
 
 import (
-	"database/sql"
 	"net/http"
 
 	"git.haelnorr.com/h/golib/jwt"
 	"github.com/pkg/errors"
 )
 
-func (auth *Authenticator[T]) RefreshAuthTokens(tx *sql.Tx, w http.ResponseWriter, r *http.Request) error {
+func (auth *Authenticator[T]) RefreshAuthTokens(tx DBTransaction, w http.ResponseWriter, r *http.Request) error {
 	aT, rT, err := auth.getTokens(tx, r)
 	if err != nil {
 		return errors.Wrap(err, "getTokens")
@@ -32,7 +31,7 @@ func (auth *Authenticator[T]) RefreshAuthTokens(tx *sql.Tx, w http.ResponseWrite
 
 // Get the tokens from the request
 func (auth *Authenticator[T]) getTokens(
-	tx *sql.Tx,
+	tx DBTransaction,
 	r *http.Request,
 ) (*jwt.AccessToken, *jwt.RefreshToken, error) {
 	// get the existing tokens from the cookies
@@ -50,7 +49,7 @@ func (auth *Authenticator[T]) getTokens(
 
 // Revoke the given token pair
 func revokeTokenPair(
-	tx *sql.Tx,
+	tx DBTransaction,
 	aT *jwt.AccessToken,
 	rT *jwt.RefreshToken,
 ) error {