Upgrade Ruby 3.4.9 -> 4.0.4

[JuanVqz]

What

Major-version Ruby hop: 3.4.9 → 4.0.4. There is no 3.5–3.9 line — Ruby went 3.4 → 4.0 directly — so this is the immediate next sequential hop. Done with the full ruby-upgrade methodology (latest-patch baseline, deprecation sweep, floor audit, boot smoke, suite as the gate). Rails stays 8.1.3 on both boots.

Thanks to the earlier DRY refactor, the version lives in one place (.ruby-version); the Gemfile (ruby file: ...) and CI (ruby-version-file:) follow it — so only .ruby-version, .tool-versions, and the Dockerfile tag needed editing.

The one real change: ostruct

The pre-bump sweep on 3.4.9 flagged it:

ostruct ... will no longer be part of the default gems starting from Ruby 4.0.0

ostruct (OpenStruct) becomes a bundled (non-default) gem in Ruby 4.0. Under Bundler, require "ostruct" is then no longer implicitly satisfied — and Rails' own boot path (zeitwerk) requires it, so the app would fail to boot on 4.0.

Fix: added gem "ostruct" to the Gemfile (harmless on 3.4, required on 4.0). Lock picks up ostruct 0.6.3.

Lockfiles

Relocked on 4.0.4: RUBY VERSION 4.0.4, PLATFORMS ruby only. BUNDLED WITH kept at 4.0.15 — a plain relock briefly dropped it to 4.0.10 (the bundler Ruby 4.0.4 ships), restored via bundle update --bundler=4.0.15 to avoid regressing the #74 pin.

Verification (host, ruby 4.0.4, Postgres 16.13) — both boots

• Deprecation sweep clean after the ostruct fix.
• bundle lock: no gem required_ruby_version conflicts.
• Dev + production boot clean, zero deprecation warnings.
• Suite green — 16 runs, 52 assertions, 0 failures.
• rubocop (42 files, and it recognizes the 4.0 target — no unknown-version error) and reek both clean.

Note

The ruby-upgrade skill's version guides stop at 3.3→3.4 (they predate 4.0), so there's no 3.4→4.0 guide yet — this hop leaned on the general methodology. Worth adding a 3.4→4.0 guide (the ostruct default-gem removal is the headline item) as a follow-up skill improvement.