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/docker-compose.yml

@@ -11,6 +11,9 @@ services:
     restart: unless-stopped
     environment:
       DATA_DIR: /data
+      # Who may moderate the contributed strip: a comma-separated email
+      # allowlist. Empty means nobody is an admin, which is the safe default.
+      ADMIN_EMAILS: ${ADMIN_EMAILS:-}
     volumes:
       # SQLite (WAL) lives on the host so a rebuild never loses accounts.
       - ./data:/data