feat: add Google Analytics 4 (GA4) integration

[Empreiteiro]

Summary

• Add GA4 as a new data source with 7 tables (page_views, traffic_sources, events, user_demographics, device_data, conversions, ecommerce) and 5 report templates
• Backend: ask_ga4.py script using JWT-based auth with service account JSON, GA4 Data API runReport, and LLM + SQLite for natural language Q&A
• Backend: ga4_router.py with test-connection, discover, and refresh-metadata endpoints
• Frontend: GA4SourceForm.tsx with service account JSON upload, property ID input, and test connection flow
• Wired into all integration points: ask router, CRUD valid types, main.py router registration, AddSourceModal, apiClient, and i18n (EN/PT/ES)

Closes #67

Test plan

• Upload a GA4 service account JSON and enter a property ID
• Test connection succeeds with valid credentials
• Connect creates source and fetches metadata (7 tables)
• Ask a question against the GA4 source and receive an LLM-generated answer with SQL
• Verify i18n strings render correctly in EN, PT, and ES
• Run npm run typecheck (passes)

🤖 Generated with Claude Code