Validate that an event's course belongs to the event group's organization

[moveson]

Summary

The hardening PR from #2235, written TDD-style (failing specs committed first). Two pieces:

1. The validation.Event now validates that its course belongs to its event group's organization. This closes every path that could mint new cross-organization relics — UI, API, imports, and DuplicateEventGroup (which copies events into a group in the same organization, so it passes naturally). The validation skips when course or event group is absent, leaving those to the presence validations.

2. The nil-course guard (carried over from closed#2234).conform_changed_course bails when the course is blank, so a nil course_id (e.g., an API PATCH with course_id: null) produces the belongs_to presence validation's 422 instead of an ArgumentError 500 from ChangeEventCourse.

Deployment note

Merge only after the staging data repair from #2235 is complete. Production audited clean, so no production data blocks this — but the eight staging relics would fail validation on their next save. (Existing invalid rows don't break reads; they'd surface only when someone edits one.)

Factory and spec fallout

The event factory previously built course and event_group independently, each minting its own organization — so every factory event was cross-organization by default. The factory now builds the course in the event group's organization (course { association :course, organization: event_group&.organization }), which fixed most of the suite automatically. Twelve spec files that explicitly paired independently-built courses and event groups now thread a single organization through their setup (a one-line change per site). Rubocop's whole-file CI check also swept the touched API controller specs (response.parsed_body conversions and the like).

Testing

• New event specs: cross-organization course is invalid (build and persisted-change paths, with the change confirmed not to persist); same-organization is valid; factory events are consistent by construction; nil course on a persisted event is invalid rather than raising. Four behavior-differentiating examples verified to fail pre-change.
• Full affected sweep: all 93 non-system spec files referencing the event factory plus the plan-effort system spec — 1,762 examples, 0 failures.
• rubocop clean on touched files.

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