Preserve one-off fee period dates

[lago-claude-ai-agent]

Add-on and one-off fee periods now keep the calendar dates expressed by offset-bearing timestamps in CSV exports and invoice detail responses, matching invoice PDFs. Frontend write-path normalization remains out of scope.

[lago-claude-ai-agent]

Held: CI is not green on this PR; not reviewed. Re-run CI, then re-review.

Checks: https://github.com/getlago/lago-api/pull/6198/checks

[lago-claude-ai-agent]

Impediment

Where I stopped: I stopped at validation and restored this revise pass's working-tree edits.

Why: The required full bundle exec rubocop exits with five unrelated existing offenses, and the required lago exec api bundle exec rspec ... command cannot run because the lago executable and API container are unavailable.

What would unblock me: Provide a checkout with a green full RuboCop baseline and the lago launcher plus API container available for the required focused specs.

[lago-claude-ai-agent]

• Read/display scope: limited calendar-date handling to add-on fee CSV exports and invoice-detail GraphQL, preserved create timestamps, and covered a negative-offset period.

[lago-claude-ai-agent]

HOLD: invoice-detail mode is stored in query-global GraphQL context, so it can leak to unrelated sibling fields in the same operation.

• Use scoped context and add a mixed-root regression spec so only descendants of the single-invoice field preserve add-on calendar dates.
• Remove the unrelated item_metadata.rb schema-comment churn.
• Amend or squash the second commit to include the required ## Context and ## Description body.

[lago-claude-ai-agent]

• Invoice detail: switched to scoped context and added a mixed-root regression proving sibling isolation.
• Schema comments: restored the unrelated item metadata annotations.
• Read/display scope: left creation, PDF output, and non-add-on periods unchanged.
• Commit format: supplied the required Context and Description sections for the driver-created follow-up.

[lago-claude-ai-agent]

HOLD: amend or squash fix(invoices): Preserve detail fee dates so that commit itself includes the required ## Context and ## Description sections; the later compliant commit does not repair the noncompliant history.