fix(security): require WEBHOOK_SECRET for the Nextcloud webhook receiver

GHSA-8vh3-g2qg-2h2c (CVSS 9.1, CWE-306): POST /webhooks/nextcloud had no
authentication when WEBHOOK_SECRET was unset (the default). The receiver
trusted the attacker-supplied user.uid and fed it to Qdrant, letting an
unauthenticated network caller delete or re-index any user's vector
embeddings.

Webhooks now require WEBHOOK_SECRET end-to-end:

- app.py: the /webhooks/nextcloud route is only mounted when WEBHOOK_SECRET
  is set; otherwise it 404s and a startup warning notes vector sync falls
  back to the polling scanner.
- webhook_receiver.py: removed the warn-and-accept fallback. No secret -> 503,
  missing/invalid bearer -> 401; the payload is never processed unauthenticated.
- webhook_routes.py / api/webhooks.py: webhook_auth_pair() raises
  WebhookSecretNotConfigured instead of returning authMethod="none"; both
  registration entry points return a clear 503 so no dead unauthenticated
  webhooks are created.

Also expose webhooks availability to the Astrolabe UI via GET /api/v1/status
("webhooks_enabled": bool), set WEBHOOK_SECRET on the docker-compose
semantic-search dev services, and update env.sample + ADR-010 / ADR-018 /
webhook-management-guide docs.

Vector sync still works without a secret via the polling scanner.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/api/management.py

@@ -243,6 +243,11 @@ async def get_server_status(request: Request) -> JSONResponse:
         "version": __version__,
         "auth_mode": auth_mode,
         "vector_sync_enabled": settings.vector_sync_enabled,
+        # Whether the /webhooks/nextcloud receiver is active. Gated on
+        # WEBHOOK_SECRET (GHSA-8vh3-g2qg-2h2c): without a secret the route is
+        # not mounted, so the Astrolabe UI can show webhooks as unavailable and
+        # vector sync falls back to the polling scanner.
+        "webhooks_enabled": bool(settings.webhook_secret),
         "uptime_seconds": uptime_seconds,
         "management_api_version": "1.0",
     }