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>

generate_data.py

@@ -7,6 +7,7 @@ fabricated TelemetrySnapshot payloads at regular intervals. Useful for:
   - Demonstrating real-time map updates without a live game client
 """
 import asyncio       # Async event loop and sleep support
+import os
 import websockets    # WebSocket client for Python
 import json          # JSON serialization of payloads
 from datetime import datetime, timedelta, timezone
@@ -32,8 +33,10 @@ async def main() -> None:
     # Starting coordinates (E/W and N/S)
     ew = 0.0
     ns = 0.0
-    # WebSocket endpoint for plugin telemetry (include secret for auth)
-    uri = "ws://localhost:8000/ws/position?secret=your_shared_secret"
+    # WebSocket endpoint for plugin telemetry. The secret must match the
+    # backend's SHARED_SECRET env var (no insecure default anymore).
+    secret = os.environ["SHARED_SECRET"]
+    uri = f"ws://localhost:8000/ws/position?secret={secret}"
     # Connect to the plugin WebSocket endpoint with authentication
     # Establish WebSocket connection to the server
     async with websockets.connect(uri) as websocket: