Sprint 13: WebCal-Abo / Kalender-Integration

- GET /api/webcal/token: erzeugt personl. Kalender-Token (einmalig)
- GET /api/webcal/{token}.ics: iCal-Feed mit Health-Erinnerungen,
  eigenen Events, Gassi-Treffen (erstellt + beigetreten), angenommenen Sittings
- RRULE für wiederkehrende Health-Einträge (intervall_tage)
- Migration: users.calendar_token (TEXT UNIQUE)
- Settings: "Kalender abonnieren" öffnet webcal://-Link + Kopier-Button
- api.js: API.webcal.getToken() / resetToken()
- SW-Cache: by-v104, APP_VER: 80

backend/static/js/api.js

@@ -400,6 +400,14 @@ const API = (() => {
     });
   }
 
+  // ----------------------------------------------------------
+  // WEBCAL
+  // ----------------------------------------------------------
+  const webcal = {
+    getToken:    () => get('/webcal/token'),
+    resetToken:  () => del('/webcal/token'),
+  };
+
   // ----------------------------------------------------------
   // ERROR-KLASSE
   // ----------------------------------------------------------
@@ -417,7 +425,7 @@ 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,
+    friends, chat, webcal,
     subscribeToPush, getLocation,
     APIError,
   };