Add OpenAPI docs, frontend, migrations, and API updates

- OpenAPI: add missing endpoints (add-from-url, subscriptions, public availability)
- OpenAPI: CalendarSubscription schema, Subscriptions tag
- Frontend app
- Migrations: count_for_availability, subscriptions_sync, user_preferences, calendar_settings
- Config, rate limit, auth, calendar, booking, ICS, availability, user service updates

Made-with: Cursor

frontend/node_modules/@babel/compat-data/data/native-modules.json

@@ -0,0 +1,18 @@
+{
+  "es6.module": {
+    "chrome": "61",
+    "and_chr": "61",
+    "edge": "16",
+    "firefox": "60",
+    "and_ff": "60",
+    "node": "13.2.0",
+    "opera": "48",
+    "op_mob": "45",
+    "safari": "10.1",
+    "ios": "10.3",
+    "samsung": "8.2",
+    "android": "61",
+    "electron": "2.0",
+    "ios_saf": "10.3"
+  }
+}