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/app/services/dispatcher.py

@@ -121,13 +121,30 @@ def _deliver_one(event_id: uuid.UUID) -> None:
             response = httpx.post(
                 settings.n8n_webhook_url,
                 json=wire_event,
+                headers={"X-Fleet-Ops-Trigger-Token": settings.n8n_webhook_trigger_token},
                 timeout=settings.n8n_http_timeout_seconds,
             )
             response.raise_for_status()
-            body = response.json()
-            success = bool(body.get("ok", True))
-            error = None if success else f"n8n reported failure: {body}"
-            error_code = None if success else "remoteReportedFailure"
+            try:
+                body = response.json()
+            except ValueError:
+                body = None
+            if isinstance(body, dict):
+                success = bool(body.get("ok", True))
+                error = None if success else f"n8n reported failure: {body}"
+                error_code = None if success else "remoteReportedFailure"
+            else:
+                # A 2xx status with a non-object (or unparsable) body means the workflow
+                # itself errored before its "Respond to Webhook" node ran -- n8n's default
+                # error response still carries a 2xx-looking status here. Treat it as a
+                # failure so the event is retried rather than lost or wrongly marked
+                # succeeded.
+                success = False
+                error = (
+                    "Unexpected non-JSON-object response from n8n "
+                    f"(status {response.status_code})"
+                )
+                error_code = "malformedResponse"
         except httpx.HTTPError as exc:
             success = False
             error = f"{type(exc).__name__}: {exc}"