feat: add support for multiple route numbers in alert notifications

- Updated FCM alert notifications documentation to include handling of multiple route numbers.
- Enhanced Telegram cancellation import logic to support multiple route numbers, ensuring correct notifications are sent for all directions.
- Modified frontend components to allow selection of multiple route numbers for alerts, improving user experience.
- Implemented backend migration to add `route_numbers` column to alerts and adjusted related database constraints.
- Introduced new utility functions for normalizing and validating route numbers in alert processing.

backend/migrations/015_alert_route_numbers.sql

@@ -0,0 +1,20 @@
+BEGIN;
+
+-- Numbers target every direction in the city. Keep route_id for older clients.
+ALTER TABLE alerts ADD COLUMN IF NOT EXISTS route_numbers TEXT[] NOT NULL DEFAULT '{}';
+UPDATE alerts a SET route_numbers = ARRAY[upper(regexp_replace(r.route_number, '[[:space:]№]', '', 'g'))]
+FROM routes r WHERE r.id=a.route_id AND cardinality(a.route_numbers)=0;
+CREATE INDEX IF NOT EXISTS idx_alerts_route_numbers ON alerts USING gin(route_numbers);
+
+-- Removing one direction must not remove a notification for the whole number.
+ALTER TABLE alerts DROP CONSTRAINT IF EXISTS alerts_route_id_fkey;
+ALTER TABLE alerts ADD CONSTRAINT alerts_route_id_fkey FOREIGN KEY(route_id) REFERENCES routes(id) ON DELETE SET NULL;
+
+-- One source publication creates one alert. A NULL alert_id preserves deletion.
+-- Legacy per-direction links stay intact until that publication is edited/reviewed.
+CREATE TABLE IF NOT EXISTS telegram_message_alerts (
+  message_id INTEGER PRIMARY KEY REFERENCES telegram_import_messages(id) ON DELETE CASCADE,
+  alert_id INTEGER UNIQUE REFERENCES alerts(id) ON DELETE SET NULL
+);
+
+COMMIT;