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>

backend/Dockerfile

@@ -18,10 +18,18 @@ RUN touch src/main.rs && cargo build --release
 # --- Runtime stage ---
 FROM alpine:3.21
 
-RUN apk add --no-cache ca-certificates ffmpeg
+# Run as an unprivileged user (defense-in-depth). The media volume mounts at
+# /media; creating it here as `app` means a fresh named volume inherits app
+# ownership and is writable without running as root. (An existing root-owned
+# volume from a prior deploy needs a one-time `chown -R app:app` — see deploy notes.)
+RUN apk add --no-cache ca-certificates ffmpeg \
+    && addgroup -S app && adduser -S -G app app \
+    && mkdir -p /media && chown app:app /media
 
 WORKDIR /app
 COPY --from=builder /app/target/release/eventsnap-backend ./
+RUN chown -R app:app /app
 
+USER app
 EXPOSE 3000
 CMD ["./eventsnap-backend"]