RSS Feed Caching for DX Calendar and Contest Calendar

Description

This PR implements a cron-based RSS feed caching system for the DX Calendar and Contest Calendar pages, following the existing pattern for other cron jobs.

Why?

Cleaner architecture and most imporant - posibility to quickly display part of the data in other parts of the website in the future. As a side benefit - on my test instance DX Calendar load time was reduced from 7s to 1.5s.

Problem

Previously, both calendar pages fetched RSS data from external sources (ng3k.com and contestcalendar.com) on every page load. This caused:

• Slow page loads when external sources were unresponsive
• Unnecessary load on external servers
• Not perfect user experience during network issues
• For DX Calendar: additional performance issues due to 4 database queries per DXpedition entry

Solution

Backend RSS Feed Caching

• New cron job (update_rss_feeds) that runs every 12 hours to fetch and cache RSS feeds - this can be simply improved to add more RSS sources in the future
• Lockfile mechanism (/tmp/.update_rss_feeds_running) to prevent concurrent executions - following other cron jobs
• XML validation before caching to ensure only valid RSS data is stored - simplified the display code
• Debug page integration for manual trigger and status monitoring

DX Calendar Performance Optimization

• New batch query function get_batch_dxcalendar_statuses() in Logbook_model
• Reduces database queries from 4×N (4 queries per DXpedition) to just 4 total queries
• Uses IN clauses to check all callsigns/DXCCs at once

Frontend Improvements

• Calendar pages now read from cache only (no external fetches)
• Role-based error messages:
  • Admins: See instruction to trigger update from Debug page
  • Regular users: See message to contact administrator

Migration

Migration 268 adds the new cron job:

• ID: update_update_rss_feeds
• Schedule: 0 */12 * * * (every 12 hours)
• Description: Update RSS feeds for DX Calendar and Contest Calendar

Testing

1. Run migration to add the cron job
2. Navigate to Debug page and trigger "RSS Feeds" update
3. Verify DX Calendar and Contest Calendar load from cache
4. Check that cache files are created in application/cache/