Add communication preference endpoints

[dgee2]

Goal

Allow users to view and update their communication preferences per category via API.

Spec reference

docs/specs/recipe-creation-target-architecture.md — §"Authenticated recipe endpoints"

Implementation notes

• GET /api/user/communication-preference — returns full list of all known categories with current isOptedIn value (defaults to false for categories with no row)
• PUT /api/user/communication-preference — batch upsert preferences; body: CommunicationPreferenceItem[] { category, isOptedIn }
• DTO: CommunicationPreferenceItem { category: string, isOptedIn: bool }

Acceptance criteria

• GET returns all known categories (even those with no row, defaulting to opted-out)
• PUT upserts each item in a single transaction

Testing

• Integration tests: get with no rows (all default false), update a category, re-get reflects update

Part of#1108