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>

Dockerfile

@@ -36,13 +36,14 @@ ARG BUILD_VERSION=dev
 ENV APP_VERSION=$BUILD_VERSION
 
 ## Default environment variables for application configuration
+## NOTE: no SHARED_SECRET default here on purpose — main.py fails closed
+## (refuses plugin connections) unless a real value arrives via compose/.env.
 ENV DATABASE_URL=postgresql://postgres:password@db:5432/dereth \
     DB_MAX_SIZE_MB=2048 \
     DB_RETENTION_DAYS=7 \
     DB_MAX_SQL_LENGTH=1000000000 \
     DB_MAX_SQL_VARIABLES=32766 \
-    DB_WAL_AUTOCHECKPOINT_PAGES=1000 \
-    SHARED_SECRET=your_shared_secret
+    DB_WAL_AUTOCHECKPOINT_PAGES=1000
 
 ## Launch the FastAPI app using Uvicorn
 CMD ["uvicorn","main:app","--host","0.0.0.0","--port","8765","--workers","1","--no-access-log","--log-level","warning"]