feat: add Stripe integration with 9 tables and 5 report templates

[Empreiteiro]

Summary

• Add Stripe as a new data source integration (source type: stripe)
• Supports 9 resource tables: customers, subscriptions, invoices, charges, products, prices, refunds, payouts, disputes
• Includes 5 built-in report templates: MRR & Revenue, Subscription Analytics, Payment Health, Customer Insights, Payout Summary
• Auth via Bearer token (Stripe Secret API Key sk_...)
• Cursor-based pagination using starting_after / has_more
• Uses LLM + SQLite pattern consistent with other integrations (REST API, etc.)
• Frontend form with test connection, resource discovery, and table selection
• i18n translations for EN, PT, ES

New files

• backend/app/scripts/ask_stripe.py - Q&A logic with Stripe API fetching, SQLite loading, and LLM interaction
• backend/app/routers/stripe_router.py - FastAPI endpoints: test-connection, discover, refresh-metadata
• src/components/StripeSourceForm.tsx - React form component with password input, test button, resource preview

Modified files

• backend/app/routers/ask.py - Route stripe source type to ask_stripe
• backend/app/routers/crud.py - Add stripe to valid source types
• backend/app/main.py - Register stripe_router
• src/components/AddSourceModal.tsx - Add Stripe to source options and render form
• src/services/apiClient.ts - Add stripeTestConnection, stripeDiscover, stripeRefreshMetadata
• src/contexts/LanguageContext.tsx - Add i18n keys for EN, PT, ES

Test plan

• Verify npm run typecheck passes (confirmed)
• Test connection with a Stripe test API key (sk_test_...)
• Verify resource discovery lists all 9 tables with sample counts
• Connect Stripe source and ask natural language questions about payments/subscriptions
• Verify report templates generate meaningful answers
• Test with empty Stripe account (no data) - should handle gracefully

Closes #66

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