feat: add generic CalDAV calendar data source

Add @aris/source-caldav package that fetches calendar events from any
CalDAV server via tsdav + ical.js.

- Supports Basic auth and OAuth via explicit authMethod discriminant
- serverUrl provided at construction time, not hardcoded
- Optional timeZone for correct local day boundaries
- Credentials cleared from memory after client login
- Failed calendar fetches logged, not silently dropped
- Login promise cached with retry on failure

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-source-caldav/fixtures/single-event.ics

@@ -0,0 +1,26 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Test//Test//EN
+BEGIN:VEVENT
+UID:single-event-001@test
+DTSTART:20260115T140000Z
+DTEND:20260115T150000Z
+SUMMARY:Team Standup
+LOCATION:Conference Room A
+DESCRIPTION:Daily standup meeting
+STATUS:CONFIRMED
+URL:https://example.com/meeting/123
+ORGANIZER;CN=Alice Smith:mailto:alice@example.com
+ATTENDEE;CN=Bob Jones;ROLE=REQ-PARTICIPANT;PARTSTAT=ACCEPTED:mailto:bob@example.com
+ATTENDEE;CN=Carol White;ROLE=OPT-PARTICIPANT;PARTSTAT=TENTATIVE:mailto:carol@example.com
+BEGIN:VALARM
+TRIGGER:-PT15M
+ACTION:DISPLAY
+DESCRIPTION:Reminder
+END:VALARM
+BEGIN:VALARM
+TRIGGER:-PT5M
+ACTION:AUDIO
+END:VALARM
+END:VEVENT
+END:VCALENDAR