Report how long a run has been going, not just how long it took

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UytviPMJbXzD8P84nLvXcq

backend/fluksio/api/routes/observability.py

@@ -12,12 +12,13 @@ from typing import Any, Literal
 
 from fastapi import APIRouter, Depends, Request
 from fastapi.concurrency import run_in_threadpool
-from pydantic import BaseModel
+from pydantic import BaseModel, model_validator
 from sqlalchemy import ColumnElement, Integer, cast, func
 from sqlalchemy import select as sa_select
 from sqlmodel import col, select
 
 from fluksio.api.deps import FlowControllerDep, SessionDep, get_current_user
+from fluksio.api.routes.runs import elapsed_ms
 from fluksio.core.config import settings
 from fluksio.flow.controller import ADVISORY_ISSUES, NodeStatus
 from fluksio.models import EngineEvent, FlowRun, MetricBucket
@@ -81,9 +82,17 @@ class RunRow(BaseModel):
     finished_at: datetime | None = None
     nodes: int
     errors: int
+    #: How long it took, or — while it is still going — how long it has been
+    #: going: the collector only writes a duration once a cascade finishes.
     duration_ms: float
     deliveries: int
 
+    @model_validator(mode="after")
+    def _running_duration(self) -> "RunRow":
+        if self.status == "running" and not self.duration_ms:
+            self.duration_ms = elapsed_ms(self.started_at)
+        return self
+
 
 class RunPage(BaseModel):
     data: list[RunRow]