feat: add actions to FeedSource interface

Add listActions() and executeAction() to FeedSource for write
operations back to external services. Actions use arktype schemas
for input validation via StandardSchemaV1.

- ActionDefinition type with optional input schema
- FeedEngine routes actions with existence and ID validation
- Source IDs use reverse-domain format (aris.location, aris.tfl)
- LocationSource: update-location action with schema validation
- TflSource: set-lines-of-interest action with lineId validation
- No-op implementations for sources without actions

Co-authored-by: Ona <no-reply@ona.com>

apps/aris-backend/src/location/service.test.ts

@@ -9,7 +9,7 @@ describe("LocationService", () => {
 		const source = service.feedSourceForUser("user-1")
 
 		expect(source).toBeDefined()
-		expect(source.id).toBe("location")
+		expect(source.id).toBe("aris.location")
 	})
 
 	test("feedSourceForUser returns same source for same user", () => {