[B6] Collection and saga completion (TMDB collections)

Backlog, viability to be confirmed.

TMDB exposes a film's collection membership (belongs_to_collection): Alien, Star Wars, Fast & Furious. That gives a bounded, non-farmable object to complete.

Why it is interesting

• A "5/9" gauge on a saga is a natural discovery objective, not a consumption counter.
• It answers the completion problem directly: a saga is a bounded set, so completing it means something, unlike "I finished one series".
• It is as much a catalogue feature as a gamification one — the gauge is useful even without an achievement attached.

To check

• Coverage and reliability of the TMDB collection data.
• Equivalents in the other domains (game series via IGDB, book series via Open Library) — probably much weaker.
• One achievement per saga would flood the catalogue; aggregate them ("3 complete sagas").

Depends on: [G2]/[G3] achievement system.

Other domains: the data is there

On the "equivalents in the other domains" point above: three sources already return what a completion gauge needs.

• Artist: MusicBrainz browses an artist's release groups (artist: artistId), for "7 / 12 Radiohead albums".
• Author: Open Library's author:"…" search, for "3 / 9 Le Guin".
• Franchise: IGDB already returns franchises.games in DETAIL_FIELDS, for "Zelda 4 / 11".

Crossed with the library, this is the saga mechanism (already shipped for video, books and games, $lib/saga.ts) applied to artists, authors and franchises.