Let a pinned minute read runs from the whole day

/observability/runs gains since/until, so the throughput chart's pin asks
the server for its minute instead of filtering a fixed recent list. This
engine writes ~60 runs a minute, so any minute but the newest read empty.

since is inclusive and until exclusive, matching the minute buckets the
charts are drawn from. Hover stays the client-side preview it was:
scrubbing a day would otherwise be a request per minute rested on.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017MeiWk3Yq12n2pTvnQWYvt

backend/app/api/routes/observability.py

@@ -97,6 +97,11 @@ def _since(hours: int) -> datetime:
     return datetime.now(timezone.utc) - timedelta(hours=hours)
 
 
+def _aware(when: datetime) -> datetime:
+    """A bound as the columns store it. A naive one is read as UTC."""
+    return when if when.tzinfo else when.replace(tzinfo=timezone.utc)
+
+
 @router.get("/summary", response_model=HealthSummary)
 async def read_summary(
     request: Request, controller: FlowControllerDep, session: SessionDep
@@ -275,14 +280,24 @@ def read_runs(
     session: SessionDep,
     flow: str | None = None,
     status: str | None = None,
+    since: datetime | None = None,
+    until: datetime | None = None,
     limit: int = 50,
 ) -> Any:
-    """Recent cascades, newest first."""
+    """Recent cascades, newest first.
+
+    ``since`` is inclusive and ``until`` exclusive, so a window of one minute
+    holds exactly the runs of the minute bucket the charts are drawn from.
+    """
     statement = select(FlowRun).order_by(col(FlowRun.started_at).desc())
     if flow:
         statement = statement.where(col(FlowRun.flow) == flow)
     if status:
         statement = statement.where(col(FlowRun.status) == status)
+    if since:
+        statement = statement.where(col(FlowRun.started_at) >= _aware(since))
+    if until:
+        statement = statement.where(col(FlowRun.started_at) < _aware(until))
     return list(session.exec(statement.limit(min(limit, 200))))