fix(api): reject invalid purchase verify skill ids

[dirtybits]

Summary

• return 404 Skill not found for malformed repository skill UUIDs on POST /api/skills/[id]/purchase/verify
• reject malformed IDs before database initialization or PostgreSQL ::uuid casts while preserving supported chain- skill IDs
• add a no-database regression and use a valid UUID fixture for the normal purchase-verification cases

Test plan

• npm test --workspace @agentvouch/web -- __tests__/api/skills-purchase-verify.test.ts --maxWorkers=1 --no-fileParallelism
• npm run format:check
• npm run lint:web
• npm run typecheck
• npm test --workspace @agentvouch/web -- --maxWorkers=1 --no-fileParallelism
• npm exec --workspace @agentvouch/web -- next build --webpack
• git diff --check

The production build passed with the existing viem Tempo dynamic-import warning and expected local DATABASE_URL static-generation fallbacks. No live purchase flow was run; this is request-boundary validation only.

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