feat: add Salesforce CRM integration

[Empreiteiro]

Summary

• Add Salesforce CRM as a new data source with full SOQL-based data fetching (API v59.0)
• Support 8 Salesforce objects: Accounts, Contacts, Opportunities, Leads, Cases, Tasks, Campaigns, Users
• Include 5 report templates: Sales Pipeline, Lead Conversion, Account Overview, Campaign ROI, Support Cases
• OAuth-based auth using access token + instance URL, with pagination via nextRecordsUrl

Changes

• New files:
  • backend/app/scripts/ask_salesforce.py — SOQL query engine, table loading, LLM + SQLite Q&A
  • backend/app/routers/salesforce_router.py — test-connection, discover, refresh-metadata endpoints
  • src/components/SalesforceSourceForm.tsx — frontend form with access token + instance URL inputs
• Updated files:
  • backend/app/routers/ask.py — route Salesforce source type to ask_salesforce
  • backend/app/routers/crud.py — add "salesforce" to valid source types
  • backend/app/main.py — register salesforce_router
  • src/components/AddSourceModal.tsx — add Salesforce option and form rendering
  • src/services/apiClient.ts — add salesforceTestConnection, salesforceDiscover, salesforceRefreshMetadata
  • src/contexts/LanguageContext.tsx — add i18n keys for EN, PT, ES

Test plan

• Verify npm run typecheck passes (confirmed)
• Test connection with a Salesforce sandbox instance
• Test object discovery returns correct counts for all 8 tables
• Test Q&A flow with sample questions from report templates
• Verify i18n strings render correctly in EN, PT, and ES

Closes #65

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