n8n: add Fleet Ops endpoint to receive workflow error reports

New POST /api/v1/integrations/n8n/workflow-error, service-token
authenticated, for the central "Fleet Ops — Workflow Error Handler"
n8n workflow to report a bounded, secret-free failure (workflow id/
name, execution id, safe error category, trigger context, correlation
id, attempt, retry action). Idempotent on execution_id via the same
audit-event precheck pattern used by /return-callback, so a
redelivered error report is not registered twice.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/schemas.py

@@ -143,6 +143,27 @@ class ScanResultOut(BaseModel):
     created: dict[str, int]
 
 
+class WorkflowErrorReportIn(BaseModel):
+    workflow_id: str = Field(max_length=120)
+    workflow_name: str = Field(max_length=200)
+    execution_id: str = Field(max_length=120)
+    failed_at: datetime
+    error_category: Literal[
+        "timeout", "authError", "connectionError", "httpError", "validationError", "unknown"
+    ]
+    error_summary: str = Field(max_length=500)
+    trigger_context: str | None = Field(default=None, max_length=200)
+    correlation_id: str | None = None
+    attempt: int = Field(default=1, ge=1, le=1000)
+    retry_action: str | None = Field(default=None, max_length=200)
+
+
+class WorkflowErrorReportResult(BaseModel):
+    status: Literal["registered", "already_registered"]
+    execution_id: str
+    occurred_at: datetime
+
+
 class ProvideFieldsRequest(BaseModel):
     fields: dict[str, str]