feat: Expose checkbox tasks and to-dos as an ICS subscription feed

[allenporter]

Summary

Expose task checkboxes (from handwritten note OCR) and native device to-dos (from the Schedule app) as a standard iCalendar (.ics) subscription link. This will allow users to subscribe to their handwritten and digital tasks in applications like Apple Calendar/Reminders, Google Calendar, Outlook, or Nextcloud.

Technical Context

1. Data Sources

We have two distinct task sources in the SQLite database:

1. Device-Native Schedule Tasks (Digital To-Dos):
   • Source Table: t_schedule_task (mapped to ScheduleTaskDO)
   • Attributes to Map: title, detail (description), due_time (due date/time), status (completed vs needsAction), and recurrence (RRule).
2. OCR Handwritten Checkboxes (Handwritten To-Dos):
   • Source Table: f_note_page_content (mapped to NotePageContentDO)
   • Attributes to Map: Parse the text_content string for checkbox patterns identified by the Gemini Vision OCR module (e.g., lines starting with •, [ ], [x], or task indicators like > and < as defined in the prompt legend).

2. Proposed Changes

• Feed Endpoint: Expose a new public (or token-authenticated) GET endpoint, e.g., /api/feed/tasks.ics?token=<user_token> or /api/users/{userId}/tasks.ics.
• ICS Generation:
  • Use the ical (or icalendar) package to generate the iCalendar feed.
  • Map retrieved tasks to standard iCalendar VTODO components.
  • Map due_time to the DUE property.
  • Map status to the STATUS property (COMPLETED, NEEDS-ACTION).
  • Include a link back to the source note/page inside the description for OCR tasks.