[Backend] Add a richer search API for discovery

[Kaylahray]

Description

The app can filter posts through the posts endpoint, but there is no dedicated discovery API for combined post, user, and leaderboard search.

User Story

As a user,
I want to search across posts and people from one query,
so that discovery feels intentional instead of fragmented.

Requirements and Context

• Files: app/app/api/posts/route.ts, app/app/api/leaderboard/route.ts, app/app/api/users/[id]/route.ts
• Add a discovery endpoint that can search posts, users, and trending topics together
• Support pagination and ranking options
• Return normalized results for the frontend search page

Suggested Implementation

// DISCOVERY API
// 1. Accept a query string and optional filters.
// 2. Search posts, users, and leaderboard entries.
// 3. Rank and paginate the combined results.
// 4. Return a single normalized payload.

Acceptance Criteria

• One API call can return combined search results.
• Results support pagination and ranking.
• The frontend can render the payload without extra reshaping.

Submission Guidelines

• Branch: feat/discovery-api
• Depends on: current Prisma models and post filters
• PR: feat(backend): add combined search and discovery endpoint

[nice-bills]

@nice-bills has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I’d like to work on this project. I’m a seasoned Fullstack developer, well-versed in using both TypeScript and JavaScript, TailwindCSS and ExpressJS for database coordination, testing, and other practical use cases. I work quickly and can deliver in a short time.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @nice-bills to this issue.