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.

compose.yaml

@@ -32,6 +32,7 @@ services:
       RAGCORE_COLLECTION: ${RAGCORE_COLLECTION:-internal-procedures}
       RAGCORE_API_TOKEN: ${RAGCORE_API_TOKEN:-}
       N8N_WEBHOOK_URL: ${N8N_WEBHOOK_URL:-http://n8n:5678/webhook/mobilityops-return}
+      N8N_WEBHOOK_TRIGGER_TOKEN: ${MOBILITYOPS_WEBHOOK_TRIGGER_TOKEN:-replace-me-n8n-webhook-trigger-token}
       N8N_CALLBACK_TOKEN: ${MOBILITYOPS_CALLBACK_TOKEN:-replace-me-n8n-callback-token}
       MCP_HUB_SERVICE_TOKEN: ${MCP_HUB_SERVICE_TOKEN:-replace-me-mcp-hub-token}
       DEMO_ORGANIZATION_NAME: ${DEMO_ORGANIZATION_NAME:-Northstar Mobility}