Add a tool to ingest mod entity art into the CDN layout

[ptrlrd]

Follow-up to the mod-asset directory (#493). This adds the tool that turns a community mod's art into the id-named layout the site falls back to.

What it does

tools/ingest_mod.py takes a mod key from data/mods.json and a source, and stages normalized art ready for CDN upload:

• Source is a loose image folder (e.g. a cloned WatcherMod repo) or a Godot .pck. A .pck is unpacked statically with GDRE Tools, the same way update.py extracts the main game, so nothing needs the game running.
• For each entity type in the mod's paths map, it converts every image to extraction/mods/<key>/<type>/<id>.webp.
• Ids come from the source filename, lowercased, matching the in-game id convention the run and live pages resolve by.
• Godot .import sidecars and non-image files are ignored.

The per-type subpaths and the source extension come from data/mods.json, so the input matches the layout the frontend expects. Output lands in extraction/, which is gitignored, so nothing binary gets committed.

Verified

Ran it against WatcherMod (github.com/lamali292/WatcherMod):

python3 tools/ingest_mod.py --key watcher --source /tmp/WatcherMod/Watcher/images
  cards: 92 image(s)
  relics: 9 image(s)
  - .../potions has no images, skipping
Done. 101 image(s) staged

The card and relic counts match the repo once the .import sidecars are excluded. The potions folder holds only subfolders and a script, so it is correctly skipped. Sample output is valid RGBA webp.

Notes

Stacked on #493 for the directory and /api/mods endpoint. The frontend wiring that resolves modded art on the live and run pages (main then beta then mod) is a separate change.

[ptrlrd]

Folded into #496, which delivers the whole modded-card feature (directory, ingest, parse, bake, backend serving, and frontend resolution) in one MR.