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>

tests/unit/test_webhook_uri.py

@@ -14,6 +14,7 @@ import pytest
 
 from nextcloud_mcp_server.auth import webhook_routes
 from nextcloud_mcp_server.auth.webhook_routes import (
+    WebhookSecretNotConfigured,
     _get_webhook_uri,
     webhook_auth_pair,
 )
@@ -126,9 +127,12 @@ def test_localhost_fallback_when_nothing_set(monkeypatch):
 
 
 @pytest.mark.unit
-def test_auth_pair_returns_none_when_secret_unset(monkeypatch):
+def test_auth_pair_raises_when_secret_unset(monkeypatch):
+    """Security (GHSA-8vh3-g2qg-2h2c): no secret => no webhook registration.
+    The helper raises instead of returning an ``authMethod="none"`` pair."""
     _patch_settings(monkeypatch, webhook_secret=None)
-    assert webhook_auth_pair() == ("none", None)
+    with pytest.raises(WebhookSecretNotConfigured):
+        webhook_auth_pair()
 
 
 @pytest.mark.unit