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.
