Warn when a private event group's times feed projections

[moveson]

Summary

Follow-up to #2229/#2231. New events default to use_for_projections = true — correct for real events being set up pre-publication (concealed groups that will go public by race day), but it silently includes new non-public test events with messy or unrealistic data in the projection stream. This PR surfaces the concealed-but-projecting combination at the three moments a user can act on it:

1. Event form — when the event's group is concealed, an inline warning callout appears under the "Use this event's times for projections" checkbox (a visible callout, not a tooltip).
2. Setup and setup summary pages — a concealed group with projectable events shows a warning callout naming those events, each linked to its edit form. This catches the case where the group was concealed after the events were created, which the event form alone would miss.
3. Take Private confirmation — when concealing a group whose events feed projections, the confirm dialog adds: "Even while private, this group's Events will continue to feed pacing plans and projections for their Courses unless you uncheck…" The existing confirm copy was split so the "Are you sure you want to proceed?" question always comes last.

The default itself is deliberately unchanged: defaulting from concealment would silently drop every normal pre-publication event out of projections after it goes public — a worse failure in the common case — and would re-couple the two concepts #2229 just separated.

Testing

• New system spec covers the setup page (warning shown for concealed + projectable; hidden when visible; hidden when concealed with all flags off), the setup summary page, and the event form (warning shown only when the group is concealed) — 6 scenarios.
• New helper spec covers the confirm text with and without the addendum.
• Neighboring suites green: visit event group setup, edit event flow — 18 examples, 0 failures.
• rubocop and erb_lint clean on touched files.

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