feat(todo): a calendar's feed read, and linked to a project in its payload (ADR 0033, D39)

The developer's decisions of 10 Nov 2026 for calendar feeds, recorded as
D39: a project links to a calendar's iCal feed, its events shown beside
tasks, read-only; each device fetches it, the address sealed in the
project's payload, the last copy kept on the device.

This lays the ground: `readIcal` reads a feed's events (title, start,
end or duration, all-day, place, cancelled, the dates a recurrence
leaves out and the occurrences changed on their own), on the device's
clock through `timezone`, and expands their recurrence with pub.dev's
`rrule` (Apache-2.0, the developer's choice); `ProjectContent` gains
`calendarUrl`, its payload staying schema 1. Nothing shows it yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/pubspec.yaml

@@ -61,6 +61,7 @@ dependencies:
   package_info_plus: ^10.2.2
   path: ^1.9.1
   path_provider: ^2.1.6
+  rrule: ^0.2.18
   serverpod_auth_idp_flutter: 4.0.4
   serverpod_flutter: 4.0.4
   serverpod_offline_sync_client: 0.0.10