feat(backend): add combined search and discovery endpoint

[Ezeh20]

Pull Request Template

Description

Adds a dedicated discovery API that returns combined search results across posts, users, and trending topics in one normalized payload.

Fixes #307

Fixes the discovery API issue for unified search across posts and people.

This change introduces /api/discovery so the frontend can make one request for mixed discovery results instead of stitching together separate post and leaderboard/user calls. It supports query filtering, pagination, and ranking options, and returns a normalized result shape the search UI can render directly without extra reshaping.

It also adds documentation for the endpoint in app/docs/discovery-api.md.

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Checklist

• I have tested my changes locally
• I have updated documentation as needed
• I have run npx prisma generate after schema changes
• I have run npx prisma migrate dev or npx prisma migrate deploy as appropriate

Notes:

• No Prisma schema changes were made in this PR, so no migration was required.
• Local verification run:
  • corepack pnpm exec vitest run tests/api/discovery.test.ts
• Prisma client generation run:
  • npx prisma generate
• Documentation added:
  • app/docs/discovery-api.md

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Post-Merge Steps for Maintainers

If this PR includes changes to the Prisma schema:

1. Run the following command to apply the migration to your database:

   npx prisma migrate deploy

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