maintenance: allow /partage through gate, fix fragment routing, add visibility table in admin

Extract shared filepond logic into src/FilepondHandler.php class.
Admin filepond endpoints delegate to the handler after AdminAuth check.
New partage filepond endpoints at /partage/actions/filepond/ verify
share_active session flag + CSRF token, no admin auth required.

JS reads filepond-base meta tag to determine endpoint path:
- Admin pages: /admin/actions/filepond (via head.php isAdmin check)
- Partage form: /partage/actions/filepond (explicit meta)

partage/index.php sets share_active = true on form render, cleans up on
successful submit. Partage process endpoint rate-limited to 30/5min per
session. No nginx changes needed — /partage/ location already handles
PHP without auth_basic.

app/storage/cache/rate_limit/f528764d624db129b32c21fbca0cb8d6.json

@@ -1 +1 @@
-[1778581157]
+[1778596263,1778596264]

app/storage/tmp/filepond/af9a22fc91fece874a0349e2597b136f/manifest.json

@@ -0,0 +1 @@
+{"queue_type":"cover","original_name":"2026-05-06-105104_hyprshot.png","mime":"image/png","ext":"png","size":170350,"session_id":"60cb0e3107c8795cc3c0d2b8c222953e","uploaded_at":"2026-05-13T12:18:50+00:00"}