refactor(webhooks): bound queue waits, route URLs through dynaconf

Addresses round-3 review feedback on PR #747:

- webhook_receiver: wrap send_stream.send() in anyio.fail_after(1.0)
  and return 503 with reason="queue full" if the queue is saturated.
  Avoids pinning the handler until NC's outbound timeout fires; the
  503 retry contract is the same as the existing "sync not running"
  branch.
- webhook_receiver: revise the compare_digest comment to match what
  the function actually guarantees — it avoids the per-character
  short-circuit of `==` but is not fully constant-time across length
  differences.
- _get_webhook_uri: read WEBHOOK_INTERNAL_URL and
  NEXTCLOUD_MCP_SERVER_URL via dynaconf so operators using
  settings.toml (rather than env vars) aren't silently routed into
  the docker/localhost fallback. Adds webhook_internal_url to
  Settings/_DEFAULTS/_field_map; nextcloud_mcp_server_url already
  existed. Docker-detection markers stay on os.getenv since they're
  container-runtime signals, not user-facing config.
- webhook_routes: sweep remaining f-string logger calls to lazy %s
  formatting per CLAUDE.md.
- client/webhooks: modernise full file's type hints to
  dict / list / | None per CLAUDE.md.

Tests:
- New test_returns_503_when_queue_is_full exercises the timeout
  branch with a saturated buffer and a shortened deadline.
- test_webhook_uri tests now patch get_settings (matching the
  auth-pair tests in the same file) instead of monkeypatching env
  vars directly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_webhook_endpoint.py

@@ -165,6 +165,33 @@ def test_returns_500_when_stream_is_closed():
     assert response.json()["status"] == "error"
 
 
+def test_returns_503_when_queue_is_full(monkeypatch):
+    """When the processor queue is saturated, the handler must time out
+    quickly with 503 instead of pinning until NC's outbound timeout fires."""
+    # Speed up the test — a 1s deadline matches production but is overkill
+    # for a unit test that's specifically exercising the timeout branch.
+    # Capture the original BEFORE patching so the override doesn't recurse
+    # into itself (the receiver imports the same anyio module object).
+    real_fail_after = anyio.fail_after
+    monkeypatch.setattr(
+        "nextcloud_mcp_server.vector.webhook_receiver.anyio.fail_after",
+        lambda _seconds: real_fail_after(0.05),
+    )
+
+    # Buffer of 1, no consumer → first send fills it, second blocks.
+    send_stream, _receive_stream = anyio.create_memory_object_stream(max_buffer_size=1)
+    send_stream.send_nowait("sentinel")  # type: ignore[arg-type]
+    app = _make_app(send_stream=send_stream)
+
+    with TestClient(app) as client:
+        response = client.post("/webhooks/nextcloud", json=_NOTE_CREATED)
+
+    assert response.status_code == 503
+    body = response.json()
+    assert body["status"] == "unavailable"
+    assert body["reason"] == "queue full"
+
+
 # --- WEBHOOK_SECRET authentication ---------------------------------------