HOLD: Add bust_cache param to fix stale featured workshops

[maebeale]

What is the goal of this PR and why is this important?

• Featured workshops on the home page can stop appearing due to a stale cache
• The workshop ID cache (1 year TTL) is invalidated via before_save, which runs inside the save transaction — if another request repopulates the cache before the transaction commits, stale data persists indefinitely

How did you approach the change?

• Added bust_cache=true URL param support to Home::WorkshopsController — deletes the cached IDs before re-fetching
• Forwarded the param through the turbo frame in _workshops.html.erb so visiting /?bust_cache=true busts the cache server-wide
• Added request spec covering the cache bust behavior
• Documented in README under "Production Maintenance"

UI Testing Checklist

• Visit / — featured workshops section loads normally
• Visit /?bust_cache=true — featured workshops appear (cache refreshed)
• Mark a workshop as featured, visit home — verify it appears (or bust cache if not)

Anything else to add?

• The caching was introduced in Refactor dashboard lazy load #977 — only workshops use this pattern; stories, community news, and events query the DB directly
• The before_save → transaction race condition is the root cause, bust_cache is a manual escape hatch

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