[Feature] Expand system metadata: dice/materials, system family, license, year

[hunter-read]

What problem does this solve?

The game_systems model captures name, description, publishers, genre, tags, is_explicit, is_system_agnostic, and cover fields — but nothing about how a system is played or lineaged. That makes it hard to browse/filter by the attributes people actually care about (what dice do I need, what family is this, is it open-licensed, how old is it).

What would you like to see?

Add the following metadata fields to game systems (editable in the system editor, displayed on the system detail view):

• Dice / Materials — the physical components a system uses. Multi-value (e.g. D20, D10, D6 pool, tarot cards, Jenga tower, playing cards, no dice). Ideally a tag-like multi-select rather than free text.
• System Family — the design lineage / engine (e.g. Powered by the Apocalypse, Havoc System, Year Zero Engine, d20 System, Forged in the Dark). Enables grouping related systems.
• License — e.g. OGL 1.0a, ORC, CC-BY, proprietary.
• Year — publication/first-release year (books.year already exists as Integer; mirror at the system level).

Notes / open questions

• Dice/Materials and System Family likely want to be curated/normalized values (so filtering works) rather than free text — worth deciding on an enum/lookup vs. tag store.
• Downstream: these should feed into system browsing/filtering (cf. [Feature] Filter the system library by tags #146 tag filtering).
• Remember to update docs/data-model.md for any schema change.

Filed for tracking.