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/models/errors.go

@@ -28,7 +28,7 @@ var (
 )
 
 func NewValidationError(detail string) *AppError {
-	return &AppError{Status: http.StatusBadRequest, Code: "VALIDATION_ERROR", Message: "Validation failed", Details: detail}
+	return &AppError{Status: http.StatusBadRequest, Code: "VALIDATION_ERROR", Message: detail}
 }
 
 func NewConflictError(detail string) *AppError {