fix: stop the prepared demo failure from degrading n8n integration health

The demo seed plants exactly one failed delivery (BK-H-0020) to demonstrate
retry and audit. Because derive_n8n_status() counted any failure, every fresh
reset pinned the n8n integration to "degraded" -- the demo showed a warning
about a prop, which tells a viewer something untrue about the automation.

The seeded failure now carries its own error code, demoScenarioTimeout, rather
than the generic connectionError a real timeout produces. No column and no
migration: last_error_code already existed, is already surfaced to the UI and is
already localizable.

- integration status splits failed into unexpected_failed and
  demo_scenario_failed; only unexpected failures may move the state. A staged
  failure alone leaves n8n operational.
- latest_failure_at is a health signal and now ignores the staged failure;
  latest_demo_scenario_at reports it separately.
- /api/v1/workflows exposes is_demo_scenario. The Automation page labels the run
  as a prepared demo scenario, explains that it is a simulated temporary failure
  that does not affect automation health, and offers a distinct "retry demo
  scenario" action. Translated in nl-BE, en-GB and fr-BE.
- the carve-out stays narrow: a real failure still degrades n8n, and a genuine
  later failure of the same event overwrites the demo code with the real one.
- the retry itself is unchanged and real: the event goes back on the outbox and
  the dispatcher delivers it to n8n like any other, so 19+1 becomes 20+0 only on
  an actual round trip. The audit records which kind of failure was retried.

Tests that assert on the seeded scenario now reseed first, since earlier test
files legitimately mutate the outbox and the suite shares one database.

Verified locally against a real PostgreSQL 16: 181 passed, ruff clean, mypy
clean (50 files), tsc clean, frontend build clean. Not deployed and not
browser-verified.

backend/app/schemas.py

@@ -266,11 +266,21 @@ class N8nIntegrationStatus(BaseModel):
     dispatch_enabled: bool
     state: Literal["disabled", "unavailable", "degraded", "operational", "no_evidence"]
     pending: int
-    delivering: int
+    #: Every failed delivery, staged and real together -- the number a viewer sees in
+    #: the run list.
     failed: int
+    #: Failures that were not planted by the demo seed. This is the only failure count
+    #: that may influence `state`.
+    unexpected_failed: int = 0
+    #: Prepared demo failures (see `app.models.outbox.DEMO_SCENARIO_ERROR_CODE`).
+    #: Present so the UI can label them instead of implying the automation is broken.
+    demo_scenario_failed: int = 0
+    delivering: int
     succeeded: int
     latest_success_at: datetime | None
+    #: Most recent *real* failure; a staged one never sets this.
     latest_failure_at: datetime | None
+    latest_demo_scenario_at: datetime | None = None
     expected_workflow_count: int
     known_workflow_count: int
     workflows: list[N8nWorkflowEvidence]
@@ -365,6 +375,10 @@ class AutomationRunOut(BaseModel):
     attempts: int
     last_error: str | None
     last_error_code: str | None
+    #: True for the deliberately seeded demo failure. The UI uses this to label the run
+    #: as a prepared scenario and to offer the demo retry, instead of presenting it as
+    #: an unexplained production error.
+    is_demo_scenario: bool = False
     occurred_at: datetime