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/utils/pgtype.go

@@ -75,6 +75,27 @@ func ToPgBool(b bool) pgtype.Bool {
 	return pgtype.Bool{Bool: b, Valid: true}
 }
 
+func ToPgInt2Ptr(i *int) pgtype.Int2 {
+	if i == nil {
+		return pgtype.Int2{Valid: false}
+	}
+	return pgtype.Int2{Int16: int16(*i), Valid: true}
+}
+
+func ToPgInt4Ptr(i *int) pgtype.Int4 {
+	if i == nil {
+		return pgtype.Int4{Valid: false}
+	}
+	return pgtype.Int4{Int32: int32(*i), Valid: true}
+}
+
+func ToPgBoolPtr(b *bool) pgtype.Bool {
+	if b == nil {
+		return pgtype.Bool{Valid: false}
+	}
+	return pgtype.Bool{Bool: *b, Valid: true}
+}
+
 func NullPgUUID() pgtype.UUID {
 	return pgtype.UUID{Valid: false}
 }