feat: implement high priority features
1. Advanced Search (backend)
- Add salaryMin, salaryMax, currency, sortBy to JobFilterQuery
- Add 5+ filters: visa, salary range, currency, language level
- Add 4 sort options: recent, salary_asc, salary_desc, relevance
2. Email Service (backend)
- Create Resend API integration (email_service.go)
- 3 HTML email templates: welcome, password_reset, application_received
- Add RESEND_API_KEY, EMAIL_FROM, APP_URL env vars
3. i18n (frontend)
- Create 4 language files: pt-BR, en-US, es-ES, ja-JP
- 100+ translation keys per language
- Covers: common, nav, auth, jobs, profile, company, footer
4. Stripe Integration (backend)
- Create payment_handler.go with checkout session creation
- Webhook handler with signature verification
- Support for checkout.session.completed, payment_intent events
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