Support location-scoped notification subscriptions (location.feed_added)

[davidgamez]

Description

location.feed_added alerts a user when a feed is added for a location they follow, so a subscription must carry the list of locations. We'll add a dedicated notification_subscription_location join table and expose the location list through the API.

Notification type in scope

Notification type	Target
location.feed_added	list of location IDs

Schema changes (docs/UserServiceAPI.yaml)

Add location_ids to the create request and to the subscription response:

CreateNotificationSubscriptionRequest:
  properties:
    # ...notification_id...
    location_ids:
      type: array
      items:
        type: string
      description: Location IDs to subscribe to. Required for location.feed_added.

NotificationSubscription:
  properties:
    # ...existing fields...
    location_ids:
      type: array
      items:
        type: string
      nullable: true
      description: Locations this subscription targets (location.feed_added).

Code changes

• POST /v1/user/subscriptions creates one subscription referencing all supplied locations and returns it with location_ids populated. _Validation is enforced in code, not the schema. Per-type schema validation would need oneOf/discriminator or JSON Schema if/then/else (3.1 only; this spec is 3.0.0), and union typing was already abandoned here due to openapi-generator limitations (see docs/DatabaseCatalogAPI.yaml SearchFeedItemResult).

DB changes

Add new relation table between user subscriptions and locations