harden: low-severity bucket — unspoofable IP, admin floor, log/SSE hygiene

The cheap, real LOWs from the review (bucket 🅰); 🅱 items and won't-fix
decisions are recorded in docs/SECURITY-BACKLOG.md.

- XFF spoofing (highest-value): client_ip now prefers an unspoofable X-Real-IP
  (Caddy `header_up X-Real-IP {remote_host}`) and otherwise takes the *rightmost*
  X-Forwarded-For token, never the client-controlled leftmost. The join cap,
  recover throttle, and admin-login floor all keyed on this. Unit-tested.
  Caddyfile also drops the dead /media/previews|originals cache matchers (the
  gateway sets its own Cache-Control) and the false "only host can download"
  comment.
- admin_login: add a hard, non-disableable rate floor (30/5min/IP) so the DB
  rate-limit master toggle can't remove brute-force protection.
- SSE: the broadcast upload-error payload no longer includes the raw error
  (absolute filesystem paths) — generic message to clients, details logged
  server-side only.
- Access logs: the request span logs the path only, never the query string, so
  the replayable media `?sig=` capability isn't written to logs.
- Reaper TOCTOU: reap_deleted now deletes rows by the ids it actually unlinked
  (DELETE … WHERE id = ANY) instead of re-evaluating the time predicate, so a
  row crossing the grace line mid-sweep can't be row-deleted without its files
  unlinked.
- Dockerfile: run the backend as a non-root user; create /media owned by `app`
  so a fresh volume is writable.
- a11y: aria-labels on the upload caption and feed search inputs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Caddyfile

@@ -5,21 +5,24 @@
     @hashed_assets path_regexp hashed /_app/immutable/.*\.[a-f0-9]{8,}\.(js|css|woff2)$
     header @hashed_assets Cache-Control "public, max-age=31536000, immutable"
 
-    # Media previews and thumbnails
-    @previews path /media/previews/* /media/thumbnails/*
-    header @previews Cache-Control "public, max-age=3600"
-
-    # Original media files (private — only host can download)
-    @originals path /media/originals/*
-    header @originals Cache-Control "private, max-age=86400"
-
     # API — never cache
     @api path /api/*
     header @api Cache-Control "no-store"
 
-    # Route API and media requests to the Rust backend
-    reverse_proxy /api/* app:3000
-    reverse_proxy /media/* app:3000
+    # Media is served by the authenticated gateway at /media/{kind}/{id}, which
+    # sets its own Cache-Control (private) and security headers per response — no
+    # Caddy-side cache rules (the old /media/previews|originals matchers were for
+    # the retired static mount).
+
+    # Route API and media requests to the Rust backend. Set X-Real-IP from the
+    # real TCP peer and overwrite any client-supplied value so the backend's
+    # rate-limit keys can't be spoofed via a forged X-Forwarded-For.
+    reverse_proxy /api/* app:3000 {
+        header_up X-Real-IP {remote_host}
+    }
+    reverse_proxy /media/* app:3000 {
+        header_up X-Real-IP {remote_host}
+    }
 
     # Everything else goes to SvelteKit frontend
     reverse_proxy frontend:3001