test+docs: tighten H1 assertions, cover secret/XFF edges, document boot & SSE-ban

Re-review follow-ups (non-blocking quality items):

Tests:
- moderation H1 specs now assert exact `→ 403` instead of bare .rejects.toThrow(),
  so a spurious 500 can no longer masquerade as "revocation worked".
- config: added case-insensitivity (upper/mixed-case placeholder) and the
  len==32/31 boundary cases for validate_secrets.
- rate_limiter: added the trailing-comma empty-entry case for client_ip (must
  fall back, not return "").
  (Backend unit tests: 35 pass.)

Docs:
- README: note that with APP_ENV=production a placeholder .env makes the app
  refuse to boot and Caddy wait unhealthy — reason is in `docker compose logs app`.
- SECURITY-BACKLOG + sse.rs comment: document that a mid-session ban does not
  tear down an already-open SSE stream (new tickets are blocked; low blast radius).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/SECURITY-BACKLOG.md

@@ -81,6 +81,12 @@ tenancy model changes.
 - Performance micro-indexes (`idx_like_user_upload`, comment pagination index) — current queries
   are sub-ms at this row count.
 - Optimistic-like in-flight guard, ownership-snapshot-at-mount, assorted copy tweaks — UX polish.
+- **Mid-session ban does not tear down an already-open SSE stream.** The live-ban check lives in the
+  `AuthUser` extractor, but the stream authenticates via ticket→session (`handlers/sse.rs::stream`),
+  so a user banned while connected keeps receiving broadcast events until their stream drops. New
+  tickets *are* blocked (`issue_ticket` uses `AuthUser`), so they cannot reconnect. Low blast radius
+  (read-only feed events, no re-subscribe); tearing live streams down would need a per-session
+  broadcast filter. Revisit only if bans must take effect within seconds.
 
 ## By-design notes (audit branch's signed-media model — see DECISION-media-auth.md)