Reject empty evaluation runs before database access

[vsolano9]

What & why

An empty evaluation currently reaches the database and can be stored as a zero-case passing run. This rejects results: [] at the request boundary so a broken CI producer cannot report a false-green gate.

Closes #3

Changes

• return HTTP 400 for an empty results array before initializing Supabase
• expose the non-empty constraint in the informational GET /api/runs response
• add a request-level Node test that starts the real Next route without database credentials and proves the guard runs before database access

Validation

• npm test — pass; repeated successfully 20 consecutive times
• npx tsc --noEmit — pass
• npm run build — pass
• git diff --check — pass
• npm ci — pre-existing failure: the committed lockfile no longer satisfies the ranged versions in package.json
• npm run lint — pre-existing failure: next lint is no longer a supported Next.js command

Checklist

• Focused change with a clear description
• Ran format / lint / tests (tests and typecheck pass; repository lint command is broken as noted above)
• Updated docs or CHANGELOG if relevant
• Linked the related issue