feat(photos): community film strip uploads + admin moderation

Backend
- photos table + upload storage under DATA_DIR/uploads (magic-byte sniffing,
  no multipart dep, SVG rejected, wx exclusive writes)
- POST/GET /api/photos, GET /api/photos/:id/file with nosniff + sandboxed CSP
- admin routes (ADMIN_EMAILS allowlist): list, delete one, clear all
- identity-keyed rate limits (login 20/15m, signup 5/h, upload 60/h)
- cookie gains Secure when the request is https (via trustProxy)
- /api/auth/me now 200 {user:null} instead of 401 when signed out

Frontend
- landing strip section: signed-in users upload straight from the reel,
  guests get a /app?auth=1 link
- /admin page: grid of uploads with delete + clear all
- nginx: nosniff / X-Frame-Options / Referrer-Policy, forward
  X-Forwarded-Proto so the API can mark cookies Secure behind TLS

Tests: docker/backend test/security.mjs (45 checks)

docker/frontend/nginx.conf

@@ -6,6 +6,12 @@ server {
   root /usr/share/nginx/html;
   index index.html;
 
+  # Static only, plus the API proxy below; a stricter CSP needs a per-app tune
+  # (canvas wasm, the QR image host) and belongs here once it is done.
+  add_header X-Content-Type-Options "nosniff" always;
+  add_header X-Frame-Options "DENY" always;
+  add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+
   gzip on;
   gzip_comp_level 5;
   gzip_min_length 1024;
@@ -29,7 +35,10 @@ server {
     proxy_set_header Host $host;
     proxy_set_header X-Real-IP $remote_addr;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-    proxy_set_header X-Forwarded-Proto $scheme;
+    # Hand the API the scheme the visitor actually used (TLS is terminated by
+    # Nginx Proxy Manager in front of this container) so it can mark the session
+    # cookie Secure. Falls back to nothing when that header is absent.
+    proxy_set_header X-Forwarded-Proto $http_x_forwarded_proto;
     client_max_body_size 4m;
   }