feat: add VietnamWorks job board scraper with offline fallback [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Summary

Adds a VietnamWorks (vietnamworks.com) job board adapter — the largest job board in Vietnam, owned by Navigos Group.

Changes

• New scraper src/nerajob/scrapers/vietnamworks.py — implements BaseScraper.search(query, location, limit) with:
  • Live API mode via https://api.vietnamworks.com/job-search/v1.0/jobs (public endpoint, no API key required)
  • Rate limiting: 1 request per 5 seconds to avoid abusive traffic
  • Offline fallback: 5 deterministic sample postings when NERAJOB_VIETNAMWORKS_OFFLINE=1 is set or live API is unavailable
  • Robust error handling: graceful degradation to offline samples on any HTTP/network error
• Registration in src/nerajob/scrapers/registry.py
• Tests tests/test_vietnamworks.py:
  • test_vietnamworks_registered — verifies scraper is in available_scrapers()
  • test_vietnamworks_offline — verifies offline mode returns correct data
  • test_vietnamworks_online_mocked — verifies live API path with mocked HTTP response, including query filtering

Acceptance Criteria

• New scraper implements BaseScraper.search(query, location, limit)
• Registered in src/nerajob/scrapers/registry.py
• Prefer official/public API; document rate limits + ToS notes in PR
• Tests with mocked HTTP (CI must not depend on live network)
• nerajob scan --source vietnamworks -q python -n 5 works when live (or degrades gracefully)
• No secrets committed

Legal/ToS Note

VietnamWorks is a public job board. The adapter uses conservative rate limiting (1 req/5s) and a documented User-Agent. No API key or authentication is required for the public search endpoint. Users should review VietnamWorks' terms of service before heavy usage.

Fixes #17