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/src/services/compression.rs

@@ -54,10 +54,16 @@ impl CompressionWorker {
                     });
                 }
                 Err(e) => {
+                    // Log the detailed error (incl. paths) server-side only; the
+                    // SSE channel is broadcast to every client, so send a generic
+                    // message — never leak absolute filesystem paths.
                     tracing::error!("compression failed for upload {upload_id}: {e:#}");
                     let _ = worker.sse_tx.send(SseEvent {
                         event_type: "upload-error".to_string(),
-                        data: serde_json::json!({ "upload_id": upload_id, "error": e.to_string() }).to_string(),
+                        data: serde_json::json!({
+                            "upload_id": upload_id,
+                            "error": "Verarbeitung fehlgeschlagen."
+                        }).to_string(),
                     });
                     let _ = Upload::set_compression_status(&worker.pool, upload_id, "failed").await;
                 }