Add cross-app integration capabilities manifest

[jaywedgeworth22]

Summary

• add token-gated /api/export/capabilities for Agentic Trading/App B integration discovery
• expose supported import slots, current import limits, market reads, analytics reads, PIT exports, and bulk snapshot table metadata without leaking secrets or peer URLs
• document the manifest in the cross-app data-sharing guide

Verification

• npm test -- --run src/export/tests/routes.test.ts
• npm run typecheck
• npm test
• npm run preview:deploy
• live preview smoke: unauthenticated /api/export/capabilities returns 401

Preview: https://congress-trade-preview.jaywedgeworth22.workers.dev

[jaywedgeworth22]

Included in the production integration merge via #113 (main commit 3317b42) and deployed to congress.trade. Closing this PR as superseded.