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/runs.py

@@ -5,11 +5,12 @@ in hours, so nothing here waits for one. The way to follow a run is to poll it
 or to listen on the flow socket, which carries its start and finish.
 """
 
+from datetime import UTC, datetime
 from typing import Any
 
 from fastapi import APIRouter, Depends, HTTPException, Request
 from fastapi.concurrency import run_in_threadpool
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, model_validator
 from sqlalchemy import func
 from sqlalchemy import select as sa_select
 from sqlmodel import Session, col, select
@@ -28,6 +29,12 @@ router = APIRouter(
 MAX_SWEEP = 1000
 
 
+def elapsed_ms(since: datetime) -> float:
+    """Milliseconds since an instant the columns stored as UTC."""
+    start = since if since.tzinfo else since.replace(tzinfo=UTC)
+    return round((datetime.now(UTC) - start).total_seconds() * 1000, 2)
+
+
 class RunCreate(BaseModel):
     params: dict[str, Any] = Field(default_factory=dict)
     seed: int | None = None
@@ -95,9 +102,17 @@ class RunRow(BaseModel):
     created_at: Any
     started_at: Any = None
     finished_at: Any = None
+    #: How long it took, or — while it is still going — how long it has been
+    #: going: a duration of its own is only written once a run finishes.
     duration_ms: float
     actor: str
 
+    @model_validator(mode="after")
+    def _running_duration(self) -> "RunRow":
+        if self.status == "running" and not self.duration_ms and self.started_at:
+            self.duration_ms = elapsed_ms(self.started_at)
+        return self
+
 
 class RunDetail(RunRow):
     result: dict[str, Any] = Field(default_factory=dict)