[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.