Upgrade to ruby 3.4

[tyranja]

Upgrade to ruby 3.4

Breaking Changes

• it is now a reserved keyword inside blocks (implicit first parameter like _1). Any code using it as a
  local variable in a block will silently change behavior or break.
• Promoted gems must be explicit in Gemfile — base64, bigdecimal, csv, drb, logger, mutex_m, ostruct,
  fiddle are no longer auto-available. You'll get LoadError if they're used without an explicit Gemfile
  entry.
• Error message format changed — single quotes instead of backticks, class name prefixed (e.g.
  'String#foo'). Any test assertions matching exception message strings may fail.
• Hash#inspect output changed — {user: 1} instead of {:user=>1}. Affects tests comparing inspect output.
• Default parser switched to Prism — slightly different error messages and edge-case parsing behavior.

Deprecation Warnings (future breaking changes)

• Frozen string literal mutations will warn. Files without # frozen_string_literal: true that mutate
  strings will emit deprecation warnings. Run tests with -W:deprecated to surface these.

Removed APIs

• Several Net::HTTP constants removed
• Refinement#refined_class removed
• Several C API functions removed (affects native gems — check gem compatibility)

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Recommended steps

1. Search codebase for it used as a local variable inside blocks
2. Add base64, bigdecimal, csv, logger, ostruct to Gemfile if used
3. Run test suite with -W:deprecated to catch frozen string literal issues
4. Check gem compatibility with Ruby 3.4 (especially any with native extensions)
5. Review any tests that assert on exception message strings

bullet changes: ⏺ The problem: in development, profile_image_link always returns avatar.png (early return on line 23-25) without ever accessing profile.image, so Bullet correctly flags the preloading as unused. But in production the preloading IS needed to avoid N+1.