Recurring events created via upsert_event show on every day instead of correct day-of-week

[chubes4]

Bug Description

When a recurring weekly event (e.g., "Monday Night Funk Jam" every Monday) is created via the AI upsert_event handler with a date range spanning multiple weeks, the calendar displays it on every single day in the range — including incorrect days (Tuesdays, Wednesdays, etc.).

Reproduction

1. Create an event via upsert_event (AI pipeline) with:
   • startDate: "2026-04-06" (Monday)
   • endDate: "2026-05-11" (Monday)
   • occurrenceDates: [] (empty)
2. View the event on the calendar
3. Event appears on all 36 days (Apr 6 through May 11) instead of only on Mondays

Root Cause

The DateGrouper::group_events_by_date() logic at inc/Blocks/Calendar/Grouping/DateGrouper.php:88-98:

if ( $has_occurrence_dates ) {
    $event_dates = $occurrence_dates;           // Use explicit dates
} elseif ( $is_multi_day ) {
    $event_dates = MultiDayResolver::get_date_range( ... );  // ALL days in range
} else {
    $event_dates = array( $start_date );        // Single day
}

When occurrenceDates is empty and startDate !== endDate, the event is treated as a continuous multi-day event (like a festival). MultiDayResolver::get_date_range() generates every calendar date between start and end — correct for festivals, wrong for weekly recurring events.

Contrast with Working Approach

The SingleRecurring handler (inc/Steps/EventImport/Handlers/SingleRecurring/SingleRecurring.php) creates one event per occurrence with startDate = endDate (single day), updated weekly by the pipeline. This works correctly.

Affected Event

• Post ID 163758 on events.extrachill.com: MONDAY NIGHT FUNK JAM at LO-Fi Brewing
• Created by: upsert_event handler (flow_id 10, pipeline_id 3)
• startDate: "2026-04-06", endDate: "2026-05-11", occurrenceDates: []
• Should only appear on 6 Mondays: Apr 6, 13, 20, 27, May 4, 11

Proposed Fix Options

Option A: Fix the upsert path

When the AI EventUpsert creates an event where:

1. startDate and endDate are different
2. The span is >7 days
3. The start and end fall on the same day of week

→ Automatically populate occurrenceDates with each recurrence date instead of treating it as a continuous multi-day range.

Option B: Add a "recurring pattern" attribute

Add a recurrencePattern block attribute (e.g., "weekly:monday") that the DateGrouper can use to expand only the correct days, similar to how iCal RRULE works.

Option C: Guard in DateGrouper

In DateGrouper, detect when a multi-day range spans weeks and the event only occurs on one day-of-week. This is fragile but would catch existing broken events.

Immediate Workaround

Manually set occurrenceDates on the block attributes to the correct dates.