Adds the server side of auto-update (issue #16, was epic #6's [H]).
- pb_migrations/1786500000_releases.js: a superuser-only 'releases' collection
(platform, variant, version, buildNumber, file, sha256, size, notes). Read is
gated in the rules — any signed-in account sees clean builds; only an
nsfwEnabled account sees adult ones — so PocketBase's native protected-file
serving hands adult bytes only to flagged accounts (no custom streaming).
- pb_hooks/update.pb.js: GET /api/update/manifest?platform=… (auth). Picks the
variant server-side from the caller's nsfwEnabled (adult) vs clean — the client
can't request adult — and returns the latest build's version/buildNumber/notes/
sha256/size + the protected downloadPath.
- scripts/publish-release.sh: uploads a built artifact as a superuser (computes
sha256 + size, multipart POST). Token or email+password via env.
- docs/auto-update-contract.md: the collection, endpoint, download flow, gating.
Migration + hook + script syntax-checked. Live verification pends deploying this
to the PB (collection auto-applies on boot, hook loads from pb_hooks/).
