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/imageenc.go

@@ -20,13 +20,6 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		ctx := r.Context()
 		start := time.Now()
 
-		// Check if encryption is configured
-		if s.encGen == nil {
-			s.respondError(w, "encrypted URLs not configured", http.StatusServiceUnavailable)
-
-			return
-		}
-
 		// Extract token from URL
 		token := chi.URLParam(r, "token")
 		if token == "" {