Feature: Gasthund-Zugang für Sitter

- sitting_subscriptions Tabelle (dog_id, owner_id, sitter_id, valid_until)
- POST/DELETE/GET /api/sitting-access — Zugang gewähren/widerrufen/auflisten
- GET /api/dogs gibt Gasthunde zurück (is_guest=True, sitting_until, owner_name)
- Diary POST erlaubt Sitter-Schreibzugang; PATCH/DELETE nur für Besitzer
- Dog-Switcher: GAST-Badge bei fremden Hunden
- Dog-Profil: Sitter-Zugang-Sektion (nur für Besitzer), Freund auswählen + Datum
- Diary Detail-View: Bearbeiten-Button für Gasthunde ausgeblendet

backend/static/js/api.js

@@ -510,6 +510,15 @@ const API = (() => {
     deactivate(id)   { return del(`/services/${id}`); },
   };
 
+  // ----------------------------------------------------------
+  // GASTHUND-ZUGANG
+  // ----------------------------------------------------------
+  const sittingAccess = {
+    grant:  (data) => post('/sitting-access', data),
+    revoke: (id)   => del(`/sitting-access/${id}`),
+    my:     ()     => get('/sitting-access/my'),
+  };
+
   const importData = {
     notestation(dogId, file) {
       const fd = new FormData();
@@ -542,7 +551,7 @@ const API = (() => {
     get, post, put, patch, del, upload,
     auth, dogs, diary, health, tieraerzte, poison,
     places, routes, walks, events, sitting, forum, lost, knigge, weather, push,
-    friends, chat, webcal, importData, sharing, widget, notifications, services, ratings,
+    friends, chat, webcal, importData, sharing, widget, notifications, services, ratings, sittingAccess,
     subscribeToPush, getLocation,
     APIError,
   };