security: enforce real plugin secret, fix proxy auth bypass, loopback DB ports, nightly backups

- SHARED_SECRET now read from env and fail-closed: unset/placeholder refuses
  ALL plugin connections (constant-time compare). The old hardcoded
  'your_shared_secret' in this public repo was no auth at all. Dockerfile
  default removed; generate_data.py reads the env var.
- SECRET_KEY fails closed at startup (main.py and agent/auth.py) instead of
  falling back to a publicly-known signing key; agent systemd unit now
  requires /etc/overlord/agent.env (no '-' prefix).
- AuthMiddleware + /ws/live: replace the 172.x source-IP trust (which every
  nginx-proxied internet request satisfied via docker-proxy — full session
  bypass and unauthenticated in-game command injection) with
  private-source AND no X-Forwarded-For, i.e. only genuinely internal
  callers (overlord-agent on the host, compose-network services). Invariant
  documented in nginx/overlord.conf: every tracker-bound location must set
  X-Forwarded-For.
- /character-stats/test endpoints gated behind admin (they upsert real rows).
- docker-compose: bind 5432/5433 to 127.0.0.1 (both DBs were internet-
  reachable; active brute-force observed in dereth-db logs).
- discord-rare-monitor: drop dead SHARED_SECRET constant.
- scripts/backup-databases.sh + docs/backups.md: nightly pg_dump of both DBs
  (telemetry/spawn hypertable data excluded), 10MB canary, umask 077,
  TimescaleDB restore procedure.
- Remove stray mangled-path css file from repo root.

Adversarially reviewed pre-deploy (3-lens workflow): ship verdict; deploy-
sequencing blockers addressed (secret staged before enforcement, exec bit
set, cron uses bash).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

agent/auth.py

@@ -12,8 +12,15 @@ import os
 from fastapi import HTTPException, Request, status
 from itsdangerous import BadSignature, SignatureExpired, URLSafeTimedSerializer
 
-# Mirror main.py:996-998
-SECRET_KEY = os.getenv("SECRET_KEY", "change-me-in-production-please")
+# Mirror main.py — and fail closed like it does: starting with a known
+# default key would let anyone forge a valid session cookie.
+SECRET_KEY = os.getenv("SECRET_KEY", "")
+if not SECRET_KEY or SECRET_KEY == "change-me-in-production-please":
+    raise RuntimeError(
+        "SECRET_KEY env var must be set (shared with dereth-tracker; see "
+        "/etc/overlord/agent.env) — refusing to start with a forgeable "
+        "session-signing key"
+    )
 SESSION_MAX_AGE = 30 * 24 * 3600  # 30 days
 _serializer = URLSafeTimedSerializer(SECRET_KEY)