fix: pace failed calendar refresh retries

tests/test_calendar_subscriptions.py

@@ -351,3 +351,58 @@ def test_refresh_discards_response_when_url_changes_in_flight(monkeypatch):
     assert row.url == "https://example.com/new.ics"
     assert row.ics_cache is None
     assert db.committed is False
+
+
+def test_failed_source_waits_until_next_interval_and_does_not_abort_batch(client, monkeypatch):
+    client = initialized(client)
+    monkeypatch.setattr(
+        "backend.calendar.validate_calendar_url",
+        lambda url: (url, "93.184.216.34", 443),
+    )
+    monkeypatch.setattr(
+        "backend.calendar.fetch_calendar",
+        lambda *args, **kwargs: FetchResult(ICS, None, None, False),
+    )
+    first = client.post(
+        "/api/v1/calendar-subscriptions",
+        json={"name": "first", "url": "https://example.com/first.ics"},
+    ).json()
+    client.post(
+        "/api/v1/calendar-subscriptions",
+        json={"name": "second", "url": "https://example.com/second.ics"},
+    )
+    attempts = []
+
+    def fetch(url, **kwargs):
+        attempts.append(url)
+        if url.endswith("first.ics"):
+            raise HTTPException(502, "upstream down")
+        return FetchResult(ICS, None, None, False)
+
+    monkeypatch.setattr("backend.calendar.fetch_calendar", fetch)
+
+    async def refresh_twice():
+        from sqlalchemy.ext.asyncio import async_sessionmaker
+
+        from backend.db import get_engine
+
+        factory = async_sessionmaker(get_engine(), expire_on_commit=False)
+        now = datetime.now(UTC) + timedelta(minutes=16)
+        async with factory() as db:
+            first_count = await refresh_due_subscriptions(
+                db, now=now, refresh_interval=timedelta(minutes=15)
+            )
+        async with factory() as db:
+            second_count = await refresh_due_subscriptions(
+                db, now=now + timedelta(minutes=1), refresh_interval=timedelta(minutes=15)
+            )
+        return first_count, second_count
+
+    assert asyncio.run(refresh_twice()) == (2, 0)
+    assert attempts == [
+        "https://example.com/first.ics",
+        "https://example.com/second.ics",
+    ]
+    listed = client.get("/api/v1/calendar-subscriptions").json()
+    failed = next(item for item in listed if item["id"] == first["id"])
+    assert failed["stale"] is True