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.

.env.example

@@ -29,6 +29,11 @@ N8N_BASIC_AUTH_ACTIVE=true
 N8N_BASIC_AUTH_USER=admin
 N8N_BASIC_AUTH_PASSWORD=change-me
 MOBILITYOPS_CALLBACK_TOKEN=replace-me-n8n-callback-token
+# Sent as the X-Fleet-Ops-Trigger-Token header when Fleet Ops calls the n8n return-
+# processing webhook, so the webhook trigger can require Header Auth instead of being
+# publicly callable by anyone who discovers the URL. Must match the value stored in
+# n8n's "Fleet Ops Webhook Trigger Token" Header Auth credential.
+MOBILITYOPS_WEBHOOK_TRIGGER_TOKEN=replace-me-n8n-webhook-trigger-token
 
 # RAGcore integration
 KNOWLEDGE_PROVIDER=demo