feat(backend): add category allowlist and price_usdc format validation

[leocagli]

Fixes #4
Fixes #5

Problem

#4 — The GET /api/services?category= route forwarded any category string directly to listServices() with no validation. Frontend types define ['search', 'weather', 'finance', 'ai', 'data', 'compute'] but the backend never enforced this, allowing arbitrary category strings to pollute on-chain data.

#5 — registerServiceOnChain() in contract.js accepted priceUsdc as a raw string with no format check. A malformed price like "abc", "", or "-1" could be written permanently on-chain.

Changes

backend/src/routes/registry.js

• Exported ALLOWED_CATEGORIES set (mirrors frontend/lib/types.ts)
• Added validation to GET /api/services?category=: unknown categories return 400 INVALID_CATEGORY with the allowed list in the error body

backend/src/lib/contract.js

• Added PRICE_USDC_RE = /^\d+(\.\d+)?$/ and ALLOWED_REGISTER_CATEGORIES set
• registerServiceOnChain() now validates before any on-chain write:
  • price_usdc must match the regex AND be positive → INVALID_PRICE_USDC
  • category must be in the allowlist → INVALID_CATEGORY

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📥 Commits

Reviewing files that changed from the base of the PR and between 6fb6da9 and 31b8b51.

📒 Files selected for processing (2)

• backend/src/lib/contract.js
• backend/src/routes/registry.js

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