Sync: crawl past Meetup events, improve parsing and debug, and expand workflow triggers

[carloshvp]

Motivation

• Improve event coverage by harvesting recent historical events from Meetup in addition to the iCal feed so the site can show recent past meetups.
• Make the sync behavior more observable and debuggable in CI by emitting diagnostics and exposing a debug flag.
• Allow validation on pull requests and avoid committing from non-push/schedule/manual contexts to reduce unexpected pushes.

Description

• Expanded GitHub Actions workflow to trigger on pull_request, set MEETUP_SYNC_DEBUG by default in the job, and only run the commit step for push, schedule, or workflow_dispatch events, and enhance the run-time stats output to show the most recent past event and URL.
• Updated README.md to document MEETUP_PAST_EVENTS_URL and MEETUP_SYNC_DEBUG and to reflect the new workflow triggers and behavior.
• Enhanced scripts/sync_meetup_events.py with debug logging, unescaping of iCal text, better JSON-LD parsing (including @graph nodes), utilities to extract event URLs from meetup HTML, a fetch_url wrapper, merging of iCal and past-event lists, and a crawl of the events/past page (and individual event pages) to supplement parsed past events.
• Added unit tests in tests/test_sync_meetup_events.py covering extract_event_urls_from_html and JSON-LD @graph parsing.

Testing

• Ran the new unit tests with python -m unittest tests.test_sync_meetup_events and the two included tests passed.
• Manual linting and basic smoke-run of scripts/sync_meetup_events.py were exercised during development to verify parsing branches (no automated failures reported).

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Codex Task