Add official DOL and EIA economic calendar feeds

Collect weekly claims and precise commercial petroleum inventory changes with official schedule exceptions, distinct reporting periods, raw evidence, and revisions.

Extend explicit history backfills without changing their default source scope. Report archive gaps and reject mismatched weeks instead of filling current values into history.

backend/tests/test_calendar.py

@@ -282,7 +282,7 @@ def test_worker_initializes_all_sources_and_drops_expired_month_status(trading):
         worker = CalendarWorker()
         worker.sources["https://www.federalreserve.gov/old-month.htm"] = {"error": "old"}
         async def unavailable(source, url, *, query=None):
-            assert len(worker.sources) == 15
+            assert len(worker.sources) == 17
             assert all(s["error"] for s in worker.sources.values())
             raise httpx.ConnectError("source unavailable")
         worker.fetch = unavailable