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/refreshtokens.go

@@ -1,7 +1,6 @@
 package hwsauth
 
 import (
-	"database/sql"
 	"net/http"
 
 	"git.haelnorr.com/h/golib/jwt"
@@ -10,7 +9,7 @@ import (
 
 // Attempt to use a valid refresh token to generate a new token pair
 func (auth *Authenticator[T]) refreshAuthTokens(
-	tx *sql.Tx,
+	tx DBTransaction,
 	w http.ResponseWriter,
 	r *http.Request,
 	rT *jwt.RefreshToken,