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.go

@@ -8,7 +8,21 @@ import (
 	"github.com/pkg/errors"
 )
 
-// Generates an access token for the provided subject
+// NewAccess generates a new JWT access token for the specified subject (user).
+//
+// Parameters:
+//   - subjectID: The user ID or subject identifier to associate with the token
+//   - fresh: If true, marks the token as "fresh" for sensitive operations.
+//     Fresh tokens are typically required for actions like changing passwords
+//     or email addresses. The token remains fresh until FreshExpireAfter minutes.
+//   - rememberMe: If true, the token is persistent (TTL="exp") and will be stored
+//     with an expiration date. If false, it's session-only (TTL="session") and
+//     expires when the browser closes.
+//
+// Returns:
+//   - tokenString: The signed JWT token string
+//   - expiresIn: Unix timestamp when the token expires
+//   - err: Any error encountered during token generation
 func (gen *TokenGenerator) NewAccess(
 	subjectID int,
 	fresh bool,
@@ -47,7 +61,19 @@ func (gen *TokenGenerator) NewAccess(
 	return signedToken, expiresAt, nil
 }
 
-// Generates a refresh token for the provided user
+// NewRefresh generates a new JWT refresh token for the specified subject (user).
+// Refresh tokens are used to obtain new access tokens without re-authentication.
+//
+// Parameters:
+//   - subjectID: The user ID or subject identifier to associate with the token
+//   - rememberMe: If true, the token is persistent (TTL="exp") and will be stored
+//     with an expiration date. If false, it's session-only (TTL="session") and
+//     expires when the browser closes.
+//
+// Returns:
+//   - tokenStr: The signed JWT token string
+//   - exp: Unix timestamp when the token expires
+//   - err: Any error encountered during token generation
 func (gen *TokenGenerator) NewRefresh(
 	subjectID int,
 	rememberMe bool,