fix(ci): make the lint gate fail for real and stop byparr leaking disk

The CI lint step was `biome check . || true`, so it could never fail: 14 real
violations were passing unnoticed. Drop the `|| true` and fix what it found.

Lint fixes, none of which change behaviour:
- give list items their natural identity instead of the array index
  (key={c} / key={char}, key={`skeleton-${i}`})
- document the two useEffect dependency lists that must keep their
  function-declaration handlers, with the reasoning that dropping them broke
  the tree and save-on-Ctrl+S once already (704e3363)
- scope the remaining noArrayIndexKey / useExhaustiveDependencies exemptions to
  the three files that need them, in biome.json instead of scattered comments

Storage, on a host that had grown to 81% disk:
- byparr starts a Firefox per request and never removes the profile it leaves in
  the container's writable layer. With no volume mounted, nothing else reclaimed
  it: 716 profiles / 6.8 GB in two days, ~1.7 GB/day. docker-prune.sh now removes
  orphaned profiles, identifying live ones by the open fd in /proc/<pid>/fd rather
  than by age, because browsers stay warm for ~27 hours here — longer than the
  leak window, so no age threshold can be both safe and useful.
- bound the build cache properly: buildx treats --max-used-space and --filter as
  mutually exclusive, so passing both silently dropped the 4 GB cap and the cache
  reached 49 GB.
- escalate to the emergency prune when / drops below 8 GB free, so the bound holds
  even if the schedule stops.
- clear multi-GB tmp_pack files left behind by a gc that was OOM-killed
  mid-repack; git only removes those on the next successful gc.
- make setup-cron.sh append instead of replacing the crontab (`crontab -`
  overwrites the whole file, which had been dropping the other scheduled jobs),
  and run the prune daily rather than weekly to match the leak rate.

Volumes are still never pruned: mariadb-turbo-data is a database.

package.json

@@ -11,8 +11,8 @@
 		"build": "pnpm assets:editor && next build --webpack",
 		"start": "next start",
 		"toolchain:check": "node scripts/check-node-toolchain.mjs",
-		"lint": "biome check . || true",
-		"biome:lint": "biome check . || true",
+		"lint": "biome check .",
+		"biome:lint": "biome check .",
 		"format": "biome format --write .",
 		"diag:permissions": "tsx scripts/diagnose-permission-page.ts",
 		"jobs:worker": "node --conditions=react-server --import tsx scripts/jobs-worker.ts",
@@ -108,4 +108,4 @@
 		"vite": "8.3.2",
 		"vitest": "5.0.3"
 	}
-}
+}