feat(quiz): hasQuiz event feature flag (frontend 2/4)

[GerardPaligot]

Summary

Part 2 of 4 of the Quiz frontend feature. Adds a hasQuiz event feature flag end-to-end so the upcoming Quiz tab can be shown only when relevant.

• Backend: new FeatureKey.Quiz; QuizQuestionEntity.findByEvent(eventId); every FeaturesActivated builder now sets hasQuiz = (FeatureKey.Quiz enabled) || (event has ≥ 1 quiz question). The flag short-circuits the DB check when the manual flag is on.
• API DTO: FeaturesActivated.hasQuiz (@SerialName("has_quiz"), default false for backward compat).
• Client cache: has_quiz column on the SQLDelight FeaturesActivated table + 4.sqm migration for existing installs; threaded through both the mobile (EventDaoSQLDelight) and wasmJs (EventDaoSettings) read/write paths.
• Shared entity: FeatureFlags.hasQuiz.

Scope

Part 2 of 4. The flag is delivered to the client's FeatureFlags here; the actual tab gating lands in Plans 3 (Android UI) and 4 (iOS UI) alongside the tab it gates. No UI changes in this PR.

Stacking

Stacked on #184 (shared quiz core) — base branch feat/quiz-frontend. It will auto-retarget to main once #184 merges.

Test Plan

• ./gradlew :backend:test — new EventFeaturesQuizTest (3 truth-table cases via getV4) + QuizQuestionEntityTest pass
• ktlint + detekt clean on backend, shared:core, shared:models
• Compiles on android, iOS (simulatorArm64), desktop, wasmJs; SQLDelight regenerates

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