fix(calendar): preserve floating/TZID semantics across CalDAV roundtrip (#782)

The CalDAV REPORT in `_search_events_by_date` unconditionally requested
server-side `<C:expand>`. Per RFC 4791 §9.6.5 the server then normalizes
every expanded DTSTART/DTEND to UTC `Z`, which destroyed two pieces of
information on the read path:

- RFC 5545 floating local times came back as fake-UTC (a `+00:00` suffix
  that did not match the stored value), so a 2:30 PM floating event was
  indistinguishable from a 14:30 UTC event in the MCP response.
- TZID-bound events lost their IANA TZID context — a "10am America/New_York"
  event came back as `14:00:00+00:00`, making it impossible for callers to
  reconstruct DST-aware recurrence semantics.

Replace `<C:expand>` with client-side recurrence expansion via the
`recurring-ical-events` library (promoted from transitive to direct dep),
so the wire response retains its original DTSTART format. Surface the
TZID parameter as new `start_tz`/`end_tz` fields on `CalendarEventSummary`.

Add an optional `timezone` (IANA name) parameter to `nc_calendar_create_event`
and `nc_calendar_update_event` so callers can pin a TZID for naive input;
the helper attaches `ZoneInfo(...)` and emits a paired `VTIMEZONE`
component. Naive input without `timezone` continues to store as RFC 5545
floating local time (with a warning logged). Offset-aware input continues
to store as UTC `Z`.

Drive-by: switch the update path's DTSTART/DTEND assignment from raw
`datetime` to `vDDDTypes(dt)` wrappers — the previous code produced invalid
iCal like `DTSTART:2026-05-14 10:00:00+00:00` for any TZ-aware update.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

uv.lock

@@ -2158,6 +2158,7 @@ dependencies = [
     { name = "python-json-logger" },
     { name = "pythonvcard4" },
     { name = "qdrant-client" },
+    { name = "recurring-ical-events" },
     { name = "starlette" },
 ]
 
@@ -2212,6 +2213,7 @@ requires-dist = [
     { name = "python-json-logger", specifier = ">=3.2.0" },
     { name = "pythonvcard4", specifier = ">=0.2.0" },
     { name = "qdrant-client", specifier = ">=1.17.0" },
+    { name = "recurring-ical-events", specifier = ">=3.8.0,<4.0" },
     { name = "starlette", specifier = "<1.0" },
 ]