feat: bookmark ("save for later") on Posts

[arturasmckwcz]

Context

Prototype feedback for fe#892 added an icon-only bookmark toggle on posts ("Save for later"). No bookmark concept exists in the backend today, and this needs to persist per-user across devices rather than live only in browser storage.

Blocked on need4deed-org/sdk#215 — per shared-rules.md, the contract must be defined and published in the SDK before this is implemented. No dependency on be#927/be#928 — bookmarks are post-only, not reply-aware.

Scope

• New PostBookmark entity: post, person, unique per (post, person)
• Endpoints: POST/DELETE /post/:id/bookmark; expose a bookmarked: boolean flag (scoped to the requesting user) on GET /post responses, and/or a `GET /post?bookmarked=true" filter for a future "saved posts" view
• Implement against the types published from feat: contract types for Post bookmarks (ApiPostBookmark*) sdk#215

Acceptance criteria

• feat: contract types for Post bookmarks (ApiPostBookmark*) sdk#215 published and this depends on the published version (not a local/linked SDK)
• Schema change confirmed necessary/semantically correct (per shared-rules.md)
• Bookmark endpoints exist and documented in swagger
• Migration replays cleanly on a fresh DB

Blocked on: need4deed-org/sdk#215
Referenced from need4deed-org/fe#969