feat(server): webhook Q&A models (WebhookQuestion/WebhookAnswer)

[GerardPaligot]

Summary

Gives the outbound webhook its own Q&A contract, decoupled from the internal QandaQuestion domain model.

• Adds WebhookQuestion / WebhookAnswer domain models (webhooks/domain) with an explicit order. order and is_correct are required (no Kotlin defaults) so kotlinx.serialization always emits them on the wire — even at 0 / false.
• Adds QandaQuestionEntity.toWebhookQuestion(order) mapper (webhooks/application/mappers) deriving order from list position; answers are indexed by their collection position.
• Retypes WebhookPayload.questions from List<QandaQuestion> → List<WebhookQuestion>, dropping the internal partnership_id / created_at from the wire format.
• HttpWebhookGateway now fetches questions ordered by created_at ASC (consistent with QandaRepositoryExposed; previously unordered) and assigns order via mapIndexed.

No DB migration, no OpenAPI change, no front-end change, and the partnership Q&A REST API is untouched.

Known limitation

Answer order reflects the entity collection's read position, not the partner's authored order — answers have no timestamp/order column and update() recreates them with random UUID PKs. Persisting true order was intentionally out of scope.

Test Plan

• WebhookQuestionTest — serialization keeps order and is_correct even at 0 / false; answers defaults to empty list
• QandaWebhookMapperTest — DB-backed: maps fields, indexes answers 0,1,2, preserves is_correct
• Full gate green: ./gradlew ktlintCheck detekt test build — 667 tests, 0 failures/errors

Note: the branch also commits the design + implementation plan under docs/superpowers/. Drop those commits if your repo prefers specs under server/specs/.

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