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>

env.sample

@@ -189,6 +189,15 @@ NEXTCLOUD_PASSWORD=
 #
 # Max queued documents (default: 10000)
 #VECTOR_SYNC_QUEUE_MAX_SIZE=10000
+#
+# Webhook receiver authentication (REQUIRED for webhook-driven sync).
+# Security (GHSA-8vh3-g2qg-2h2c): the /webhooks/nextcloud receiver trusts the
+# user id in the payload and feeds it to Qdrant, so it must be authenticated.
+# When WEBHOOK_SECRET is unset the route is NOT mounted and vector sync falls
+# back to the polling scanner. When set, the receiver requires
+# `Authorization: Bearer <secret>` and webhook registration injects it.
+# Generate one with: python -c "import secrets; print(secrets.token_urlsafe(32))"
+#WEBHOOK_SECRET=
 
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