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/core/config.py

@@ -23,6 +23,7 @@ class Settings(BaseSettings):
     ragcore_api_token: str = ""
     ragcore_http_timeout_seconds: float = 5.0
     n8n_webhook_url: str = "http://n8n:5678/webhook/mobilityops-return"
+    n8n_webhook_trigger_token: str = "replace-me-n8n-webhook-trigger-token"
     n8n_callback_token: str = "replace-me-n8n-callback-token"
     n8n_dispatch_enabled: bool = True
     n8n_dispatch_interval_seconds: float = 3.0