feat(orchestrator): authoritative failure-count badge endpoint (DEBT-042)

New GET /api/v1/orchestrator/events/stats?since=1h&success=false&kind=...
backed by repo.count_orchestrator_failures(since_ts, kind), which
counts failed rows across both orchestrator_events and
orchestrator_emails since the cutoff.

Window parser accepts ^\d+[smhd]$, capped at 7d. Today only
success=false is accepted on this surface so the endpoint isn't
accidentally repurposed before the next consumer is properly
designed.

Orchestrator.tsx polls the endpoint on mount + every 30 s and
renders the authoritative DB-derived count instead of deriving from
the in-memory SSE buffer + one paginated page (which silently
excluded failures older than the local window).

decnet/web/router/orchestrator/api_event_stats.py

@@ -0,0 +1,99 @@
+"""GET /api/v1/orchestrator/events/stats — authoritative failure count.
+
+The dashboard's failure-count badge previously derived its number from
+the in-memory SSE buffer + a single paginated page (capped at 500 +
+limit rows). On busy fleets, failures older than the local window
+were silently excluded and the badge read low — see DEBT-042. This
+endpoint returns the real count straight from the DB so the badge
+matches reality.
+"""
+from __future__ import annotations
+
+import re
+from datetime import datetime, timedelta, timezone
+from typing import Any, Optional
+
+from fastapi import APIRouter, Depends, HTTPException, Query
+
+from decnet.telemetry import traced as _traced
+from decnet.web.dependencies import repo, require_viewer
+
+router = APIRouter()
+
+_SINCE_RE = re.compile(r"^(\d+)([smhd])$")
+# Bounded to avoid unintentionally-expensive scans. 7d covers the
+# operator UX use case (failure-count badge) while still returning
+# in O(index seek + count).
+_MAX_SINCE = timedelta(days=7)
+
+
+def _parse_since(s: str) -> timedelta:
+    m = _SINCE_RE.match(s)
+    if not m:
+        raise HTTPException(
+            status_code=422,
+            detail="since must match ^(\\d+)[smhd]$ (e.g. '15m', '1h', '24h', '7d')",
+        )
+    value, unit = int(m.group(1)), m.group(2)
+    if value <= 0:
+        raise HTTPException(status_code=422, detail="since must be > 0")
+    delta = {
+        "s": timedelta(seconds=value),
+        "m": timedelta(minutes=value),
+        "h": timedelta(hours=value),
+        "d": timedelta(days=value),
+    }[unit]
+    if delta > _MAX_SINCE:
+        raise HTTPException(
+            status_code=422,
+            detail=f"since exceeds maximum window of {_MAX_SINCE}",
+        )
+    return delta
+
+
+@router.get(
+    "/orchestrator/events/stats",
+    tags=["Orchestrator"],
+    responses={
+        401: {"description": "Could not validate credentials"},
+        403: {"description": "Insufficient permissions"},
+        422: {"description": "Validation error"},
+    },
+)
+@_traced("api.orchestrator.events.stats")
+async def orchestrator_event_stats(
+    since: str = Query("1h", description="Window relative to now, e.g. '15m', '1h', '24h'."),
+    success: Optional[bool] = Query(
+        None,
+        description="If set, restrict the count to rows with this success value.",
+    ),
+    kind: Optional[str] = Query(
+        None, pattern="^(traffic|file|email)$",
+    ),
+    user: dict = Depends(require_viewer),
+) -> dict[str, Any]:
+    """Aggregate counts for the orchestrator activity feed.
+
+    Today only the failure-count badge consumes this surface, so the
+    only supported aggregate is ``success=false`` (everything else is
+    rejected — ``success=true`` and the unfiltered total can be served
+    by the existing ``count`` on the list endpoint without a window
+    filter, and we'd rather not paint ourselves into a corner before
+    the next consumer shows up).
+    """
+    if success is not False:
+        raise HTTPException(
+            status_code=422,
+            detail="only success=false is supported on this surface today",
+        )
+    delta = _parse_since(since)
+    since_ts = datetime.now(timezone.utc) - delta
+    count = await repo.count_orchestrator_failures(
+        since_ts=since_ts, kind=kind,
+    )
+    return {
+        "since": since,
+        "success": success,
+        "kind": kind,
+        "count": count,
+    }