feat(calendar): list external read-only (subscribed) calendars

list_calendars only kept PROPFIND responses whose resourcetype was
<c:calendar>, so external subscriptions (webcal/ICS feeds) — exposed by
Nextcloud as <cs:subscribed> collections — were silently dropped and never
appeared in nc_calendar_list_calendars (issue #830).

- Construct the calendar DAV client with `X-NC-CalDAV-Webcal-Caching: On` so
  Nextcloud exposes subscriptions as queryable CachedSubscription calendars,
  making their events readable through the existing event/search tools (the
  same mechanism Evolution/KDE desktop clients use).
- In list_calendars, override that header to "Off" so subscriptions are
  returned as cs:subscribed collections with their cs:source href, then parse
  responses whose resourcetype is c:calendar OR cs:subscribed. Subscriptions
  are reported with read_only=True and their source feed URL; their color is
  read from the Apple iCal namespace as a fallback.
- Send the custom PROPFIND markup via `body=` instead of `props=`: under
  caldav 3.x `props=` expects a list of property names and discards a raw XML
  string, producing an empty <prop/>. This also restores display_name,
  description and color which were previously falling back to defaults.
- Add `read_only` and `source` fields to the Calendar model.

Adds unit tests for subscription parsing, the body/header wiring, and the
webcal-caching header; extends the integration test to assert the new fields.

tests/client/calendar/test_calendar_operations.py

@@ -89,8 +89,18 @@ async def test_list_calendars(nc_client: NextcloudClient):
         # Optional fields
         assert "description" in calendar
         assert "color" in calendar
+        # External subscription metadata (issue #830): always present, with
+        # read_only=True / a source URL for subscribed calendars.
+        assert "read_only" in calendar
+        assert isinstance(calendar["read_only"], bool)
+        assert "source" in calendar
 
-        logger.info("Calendar: %s - %s", calendar["name"], calendar["display_name"])
+        logger.info(
+            "Calendar: %s - %s (read_only=%s)",
+            calendar["name"],
+            calendar["display_name"],
+            calendar["read_only"],
+        )
 
 
 async def test_create_and_delete_event(