Server: faster releases (cache hashes), per-version skip, longer signed-URL TTL

- gate.php : ETag now derived from size+mtime instead of md5_file($zip).
  Previously the gate hashed the entire 14 GB ZIP on every HEAD/GET call
  (including each of 8 parallel segments) → 30s-5min preparation lag for
  clients before the first byte. Now <1ms per request.
- SignManifest : disk-backed cache for SHA-256 keyed by (path,size,mtime).
  Re-signing 5×14 GB versions used to take ~25 min, now ~1s when nothing
  changed. New "Force re-hash" toggle in admin to ignore the cache.
- versions.php : per-row "🔁 Hash" button to sign a single version, plus
  a "⚙ Hash" dropdown to toggle hashAlgorithm:none for builds where the
  user accepts skipping client-side verification (manifest stays signed
  Ed25519, only the per-ZIP SHA-256 verification is bypassed).
- DownloadUrl.php : signed-URL TTL bumped 1h → 6h to cover slow ADSL users
  who need >1h to finish a 14 GB download.
- .gitignore : track server/builds/.htaccess + gate.php (still ignore the
  actual ZIP/exe binaries).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -30,8 +30,13 @@ ASTERION_VR/
 
 # Server-side secrets — never commit (config.example.php sert de template)
 server/api/config.php
+# Le contenu de server/builds/ est ignoré (gros ZIPs / launcher.exe), mais on
+# garde le code (.htaccess, gate.php, .gitkeep) sous version.
 server/builds/*
 !server/builds/.gitkeep
+!server/builds/.htaccess
+!server/builds/gate.php
+!server/builds/launcher/.gitkeep
 
 # Plans
 .claude/