docs+fix: audit live n8n state, require auth on the return webhook

Inspected the shared n8n instance (n8n.itworx.tech) live: both existing
Fleet Ops workflows are genuinely active and structurally match the repo,
but the shared X-Service-Token secret was stored as plaintext literal
text in both HTTP Request nodes (exportable in the clear), and the
production return webhook had n8n-level Authentication set to "None"
(publicly callable by anyone who discovered the URL). Findings recorded
in docs/live-ai-integration/n8n-current-state.md.

Fixed on the n8n side (both workflows published): the shared token now
lives in a single Header Auth credential instead of two literal copies;
the return webhook now requires a second, distinct Header Auth
credential.

Fixed on the Fleet Ops side to match: the outbox dispatcher now sends
the new X-Fleet-Ops-Trigger-Token header (new
MOBILITYOPS_WEBHOOK_TRIGGER_TOKEN setting) when calling the webhook.
Live-verified against the real webhook: a request with no header is now
rejected (403); a request with the correct header passes n8n's auth and
reaches Fleet Ops's own business logic.

That same live test also surfaced a real robustness gap: an n8n
execution that errors before its "Respond to Webhook" node runs can
still answer with a 2xx status and an empty body, which made
response.json() raise an uncaught exception, potentially leaving the
outbox event stuck in "delivering". Now treated as an explicit,
retryable failure (error_code=malformedResponse), with a regression
test reproducing the exact case.

backend/tests/test_dispatcher.py

@@ -67,7 +67,7 @@ def test_deliver_one_success(monkeypatch):
     event_id = _make_pending_event("MO-002")
     dispatcher._claim_due_events()
 
-    def fake_post(url, json, timeout):
+    def fake_post(url, json, headers, timeout):
         return SimpleNamespace(
             raise_for_status=lambda: None,
             json=lambda: {"ok": True, "event_id": str(event_id), "result": {}},
@@ -88,7 +88,7 @@ def test_deliver_one_failure_schedules_retry(monkeypatch):
     event_id = _make_pending_event("MO-003")
     dispatcher._claim_due_events()
 
-    def fake_post(url, json, timeout):
+    def fake_post(url, json, headers, timeout):
         raise dispatcher.httpx.ConnectError("simulated connection failure")
 
     monkeypatch.setattr(dispatcher.httpx, "post", fake_post)
@@ -102,11 +102,38 @@ def test_deliver_one_failure_schedules_retry(monkeypatch):
     assert event.last_error_code == "connectionError"
 
 
+def test_deliver_one_treats_empty_2xx_body_as_failure(monkeypatch):
+    # Reproduces a real failure mode found while live-validating the n8n webhook auth
+    # fix: a workflow that errors internally before its "Respond to Webhook" node runs
+    # can still answer with a 2xx status and an empty body. response.json() on that body
+    # raises json.JSONDecodeError -- this must be treated as a retryable failure, not an
+    # unhandled exception that leaves the event stuck in "delivering" forever.
+    event_id = _make_pending_event("MO-005")
+    dispatcher._claim_due_events()
+
+    def fake_post(url, json, headers, timeout):
+        def raise_json_error():
+            raise ValueError("Expecting value: line 1 column 1 (char 0)")
+
+        return SimpleNamespace(
+            raise_for_status=lambda: None, json=raise_json_error, status_code=200
+        )
+
+    monkeypatch.setattr(dispatcher.httpx, "post", fake_post)
+    dispatcher._deliver_one(event_id)
+
+    event = _get_event(event_id)
+    assert event.delivery_status == "pending"
+    assert event.attempts == 1
+    assert event.next_attempt_at is not None
+    assert event.last_error_code == "malformedResponse"
+
+
 def test_deliver_one_exhausts_attempts_to_failed(monkeypatch):
     event_id = _make_pending_event("MO-004")
     settings = get_settings()
 
-    def fake_post(url, json, timeout):
+    def fake_post(url, json, headers, timeout):
         raise dispatcher.httpx.ConnectError("still down")
 
     monkeypatch.setattr(dispatcher.httpx, "post", fake_post)
@@ -149,7 +176,7 @@ def test_deliver_one_handles_malformed_payload_without_getting_stuck(monkeypatch
     finally:
         db.close()
 
-    def fake_post(url, json, timeout):
+    def fake_post(url, json, headers, timeout):
         raise AssertionError("must not attempt delivery with a malformed payload")
 
     monkeypatch.setattr(dispatcher.httpx, "post", fake_post)
@@ -229,7 +256,7 @@ def test_run_dispatch_cycle_recovers_a_stale_lease_before_claiming(monkeypatch):
     finally:
         db.close()
 
-    def fake_post(url, json, timeout):
+    def fake_post(url, json, headers, timeout):
         return SimpleNamespace(
             raise_for_status=lambda: None,
             json=lambda: {"ok": True, "event_id": str(event_id), "result": {}},
@@ -245,7 +272,7 @@ def test_run_dispatch_cycle_recovers_a_stale_lease_before_claiming(monkeypatch):
 def test_run_dispatch_cycle_end_to_end(monkeypatch):
     event_id = _make_pending_event("MO-005")
 
-    def fake_post(url, json, timeout):
+    def fake_post(url, json, headers, timeout):
         return SimpleNamespace(
             raise_for_status=lambda: None,
             json=lambda: {"ok": True, "event_id": str(event_id), "result": {}},