fix: validate calendar event date attribution

[anish749]

Problem

Calendar events are currently attributed to dates using DatesForStorage(), which should handle multi-day events by returning all dates the event spans. However, the original bug description states "Right now we used a start date" — indicating there's confusion or a potential edge case with how timed events' end dates are handled.

Key concern: For timed events (not all-day events), the current implementation includes the end date as inclusive. This creates a semantic ambiguity:

• An event ending at 2026-04-09T00:00:00Z (midnight) is still attributed to April 9
• This may cause events to appear on dates they technically end at rather than occur on

Proposed Solution

1. Clarify end date semantics for timed events: The end date should be treated as exclusive (like all-day events already are per Google Calendar API), so events ending at midnight are not attributed to that final date.

2. Add validation tests: Create test cases that verify:

   • Multi-day all-day events span the correct date range
   • Timed events spanning midnight are attributed correctly
   • Events with ambiguous end times (midnight boundaries) are handled consistently

3. Document the convention: Add doc comments clarifying that calendar events are stored across all dates they occur on, with consistent end date semantics.

Example

• All-day event April 7-9: should be on April 7, 8 (April 9 is exclusive) ✓ Already correct
• Timed event April 7 20:00 to April 9 00:00: should be on April 7, 8 (April 9 00:00 means event ended, not started)
• Timed event April 7 20:00 to April 9 06:00: should be on April 7, 8, 9 ✓ Already correct

This ensures multi-day events are correctly attributed to the dates they actually occupy.