Design review: future URL validation API integration

[codethor0]

Current state

• Batch 6D added offline helper: backend/app/services/job_url_validation.py
• Unit tests exist; no network calls
• Helper is not wired to backend API routes or report intake

Design decisions needed

1. Where should validation run?
   • Google Sheet import
   • Backend report create endpoint
   • Moderation review queue
   • All three (with consistent rules)
2. What happens on invalid URLs at intake time?
3. Policy for controlled validation against company career pages (future; requires separate approval)

Constraints

• No schema or API changes until maintainer design is approved
• Contributors should extend offline helpers and tests first (see Issue Contributor onboarding: job URL validation pipeline #1)