Adds the Collections addon backend: a server-wide-per-user library of saved
places, independent of any trip, with multiple named lists, an idea/want/visited
status, and Vacay-style fusion invitations to share a list with other users.
- Data: collection / collection_members / collection_places / collection_place_tags
tables (+ migration and baseline schema). Saved places carry the owner plus a
nullable saved_by so a member deleting their account can't drop shared content.
- Service: list + place CRUD with owner-or-accepted-member visibility, dedup,
status, save-from-trip and copy-to-trip (reusing the trip copy column list),
and the full fusion-invitation state machine mirrored from vacay (send / accept
/ decline / cancel / leave) with a websocket broadcast and an invite
notification. Deleting a list snapshots its members and notifies them.
- NestJS module + addon guard (404 before auth), registered in the app module.
- Widens the place photo cache reference check to count collection places so the
nightly sweep no longer evicts photos a saved place still uses.
- collection_invite notification wired across all 22 locales.
Maurice