feat: Phase 2 security audit fixes + script execution platform

Security (P0/P1/P2):
- WebSSH: dedicated short-lived token, server_id binding, 4003 close code
- Remove /api/agent/exec from control plane (RCE surface eliminated)
- Global JWT middleware (JwtAuthMiddleware) with install-mode bypass
- decrypt_value: failure returns None, never leaks ciphertext
- Telegram: sanitize_external_message strips sensitive fields
- Agent auth: startup enforces non-empty API key, compare_digest
- Credentials: password_set bool flag, no plaintext in API responses
- audit_log: writes admin_username + ip_address on all CUD ops
- Install lock: all /api/install/* except GET /status return 403 post-install
- WebSocket dedup: publish once, subscribers deliver locally

Script execution platform:
- script_jobs.py / script_job_callback.py: async batching and callback
- script_execution_store.py / script_callback_rate.py: Redis-backed state
- script_execution_flush.py: background flusher to MySQL
- scripts.html / script-executions.html: full execution UI
- agent_url.py: centralised URL builder

Frontend:
- All 13 pages migrated from CDN to /app/vendor/ (no external deps)
- vendor/: alpinejs, tailwindcss-browser, xterm, xterm-addon-*, qrious
- Dashboard WebSocket real-time alerts; 8h JWT session timeout

Tests:
- test_security_unit.py: 15 unit tests (JWT, sanitize, vendor, install 403)
- test_api.py: env-configurable admin credentials

Co-authored-by: Cursor <cursoragent@cursor.com>

server/application/services/server_service.py

@@ -52,7 +52,8 @@ class ServerService:
         results = {}
 
         async def _check_one(server: Server) -> tuple:
-            url = f"http://{server.domain}:{server.agent_port}/health"
+            from server.infrastructure.agent_url import agent_url
+            url = agent_url(server.domain, server.agent_port, "/health")
             try:
                 async with httpx.AsyncClient(timeout=5.0) as client:
                     headers = {}