feat: contract types for Post search endpoint (ApiPostSearch*)

[arturasmckwcz]

Context

need4deed-org/be#930 needs a search endpoint covering posts, replies, people, and opportunities for the Posts page rebuild (need4deed-org/fe#969 epic). Per shared-rules.md, the API contract must be defined here first.

Scope

• ApiPostSearchParams — { q: string } (naming of the endpoint/params TBD with be#930)
• ApiPostSearchResult — matching posts, each including/highlighting the matched reply if the match was in a reply (depends on the reply shape from sdk#213)
• Confirm search scope (which fields, which entities) with product before finalizing the response shape

Acceptance criteria

• Search scope confirmed with product
• Types added and exported from the SDK
• Version bumped and published to npmjs.com

Blocks: need4deed-org/be#930
Depends on reply shape from: #213
Referenced from need4deed-org/fe#969