feat: search endpoint for Posts feed (posts, replies, people, opportunities)

[arturasmckwcz]

Context

Prototype feedback for fe#892 added a search bar covering posts, replies, people, and opportunities, plus a "People widget" to filter the feed by a single community member. Filtering by one already-known person is doable today against existing person endpoints, but free-text search across post/reply text (and matching people/opportunity names) needs a dedicated backend query rather than client-side filtering of the paginated feed.

Blocked on need4deed-org/sdk#216 (contract-first per shared-rules.md) and #927 (scope includes searching reply text, so the Reply entity must exist first).

Scope

• GET /post/search?q= (naming TBD, confirm against sdk#216) — searches post + reply text, and person/opportunity names, returns matching posts (with the matched reply included/highlighted)
• Keep the implementation simple (e.g. Postgres ILIKE) unless data volume already warrants more
• Implement against the types published from feat: contract types for Post search endpoint (ApiPostSearch*) sdk#216

Acceptance criteria

• feat: contract types for Post search endpoint (ApiPostSearch*) sdk#216 published and this depends on the published version (not a local/linked SDK)
• Search scope (which fields, which entities) confirmed with product before implementation
• Endpoint documented in swagger

Blocked on: need4deed-org/sdk#216, #927
Referenced from need4deed-org/fe#969