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/db/repository.py

@@ -1131,6 +1131,17 @@ class BaseRepository(ABC):
         """Total orchestrator-event rows, optionally filtered by kind."""
         raise NotImplementedError
 
+    async def count_orchestrator_failures(
+        self,
+        *,
+        since_ts: Any,
+        kind: Optional[str] = None,
+    ) -> int:
+        """Count failed orchestrator activity since *since_ts*, across
+        both event + email tables. Backs the dashboard's failure-count
+        badge (DEBT-042)."""
+        raise NotImplementedError
+
     async def prune_orchestrator_events(self, per_dst_cap: int = 10000) -> int:
         """Trim per-``dst_decky_uuid`` rows to a cap. Returns deleted count.