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/overlord-agent.service

@@ -20,8 +20,10 @@ WorkingDirectory=/home/erik/MosswartOverlord
 # HOME explicitly set so claude reads /var/lib/overlord-agent/.claude/*
 # instead of trying /home/erik/.claude/* (which is now 0700, locked out).
 Environment="HOME=/var/lib/overlord-agent"
-# Secrets file (root:overlord-agent 0640).
-EnvironmentFile=-/etc/overlord/agent.env
+# Secrets file (root:overlord-agent 0640). REQUIRED (no leading '-'):
+# a missing secrets file must abort startup, not fail open — auth.py also
+# refuses to start without SECRET_KEY.
+EnvironmentFile=/etc/overlord/agent.env
 # Run inside the venv populated by install.sh.
 ExecStart=/home/erik/MosswartOverlord/agent/.venv/bin/python -m agent.service
 Restart=on-failure