Remove runtime nil checks for always-initialized components

Since signing_key is now required at config load time, sessMgr, encGen,
and signer are always initialized. Remove unnecessary nil checks that
were runtime failure paths that can no longer be reached.

- handlers.go: Remove conditional init, always create sessMgr/encGen
- auth.go: Remove nil checks for sessMgr
- imageenc.go: Remove nil check for encGen
- service.go: Require signing_key in NewService, remove signer nil checks
- Update tests to provide signing_key

internal/handlers/handlers.go

@@ -95,22 +95,21 @@ func (s *Handlers) initImageService() error {
 	s.imgSvc = svc
 	s.log.Info("image service initialized")
 
-	// Initialize session manager and URL generator if signing key is configured
-	if s.config.SigningKey != "" {
-		sessMgr, err := session.NewManager(s.config.SigningKey, !s.config.Debug)
-		if err != nil {
-			return err
-		}
-		s.sessMgr = sessMgr
-
-		encGen, err := encurl.NewGenerator(s.config.SigningKey)
-		if err != nil {
-			return err
-		}
-		s.encGen = encGen
-
-		s.log.Info("session manager and URL generator initialized")
+	// Initialize session manager (signing key is validated at config load time)
+	sessMgr, err := session.NewManager(s.config.SigningKey, !s.config.Debug)
+	if err != nil {
+		return err
 	}
+	s.sessMgr = sessMgr
+
+	// Initialize encrypted URL generator
+	encGen, err := encurl.NewGenerator(s.config.SigningKey)
+	if err != nil {
+		return err
+	}
+	s.encGen = encGen
+
+	s.log.Info("session manager and URL generator initialized")
 
 	return nil
 }