feat(freelancer-discovery): DB-backed API for issue #121

[iyanumajekodunmi756]

Implements the Freelancer Discovery API described in #121:

GET /api/freelancers
?q=&skills=react,nodejs&minRating=4&page=2&limit=20
&sort=rating&order=desc
GET /api/freelancers/

Highlights:

• lib/freelancerDiscovery.ts encapsulates the SQL builder, pagination, sort/filter validation, and row -> API shape mapping.
• Single-template WHERE fragment with <null/0 IS 0 OR <pred>> lets the query issue exactly one DB round-trip per request, with every value bound as a Postgres parameter (no runtime SQL interpolation of the sort column or column name).
• ORDER BY is a fully-static switch on (sort, order) with literal ASC/DESC inlined per branch.
• Backward compatible with the existing /freelancers page: legacy ?rating= alias accepted, response shape unchanged, ?skills accepts both repeating and comma-separated values.
• Structured error responses ({error, code}) for invalid query params and downstream failures (400 / 503).
• GET /api/freelancers/[id] returns { freelancer, reputation } with reputation loaded best-effort; null on lookup failure so the page still renders.
• scripts/010-freelancer-discovery-indexes.sql adds a GIN index on users.skills (skill-overlap + ILIKE search), a btree on users.rating DESC, and a partial index on user_type IN (freelancer, both).
• 22 unit tests covering parse, validation, list/detail happy paths, pagination, fallback COUNT, validation errors, and 503 on DB error.

closes #121

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