refactor: merge internal/sync into internal/garmin, regroup api files and routes

Garmin auth/sync routes move under /api/garmin/*; sync.Service becomes
garmin.Sync with garmin.SyncConfig/ClientConfig; applog becomes
internal/log; the test mock moves into the garmin package as MockClient
(breaking the test-only import cycle the merge created); stale test
URLs and type names updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/api/profile.go

@@ -61,7 +61,7 @@ func (s *Server) handleUpdateProfile(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return
 	}
-	client, err := s.garminFor(r.Context(), userID)
+	client, err := s.clientFor(r.Context(), userID)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return
@@ -98,6 +98,6 @@ func (s *Server) handleDeleteProfile(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return
 	}
-	s.removeGarminClient(userID)
+	s.removeUserClient(userID)
 	w.WriteHeader(http.StatusNoContent)
 }