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

internal/api/openapi/specs/base.json

@@ -32,7 +32,8 @@
     { "name": "Contacts", "description": "Contact management" },
     { "name": "Availability", "description": "Calendar availability queries" },
     { "name": "Booking", "description": "Public booking links and reservations" },
-    { "name": "ICS", "description": "ICS calendar import and export" }
+    { "name": "ICS", "description": "ICS calendar import and export" },
+    { "name": "Subscriptions", "description": "Calendar subscriptions (external iCal feeds)" }
   ],
   "security": [
     { "BearerAuth": [] },