Add app user secrets SDK module

[arosenan]

Summary

• add typed appUserSecrets methods for listing, setting, deleting, and reading user-owned credentials
• send both the app-user token and backend-function service token for protected reads
• reject reads outside createClientFromRequest backend contexts and validate definition keys client-side
• require credential re-entry after an access-policy change; app code has no silent consent method

Testing

• npm run build
• npm run lint
• npm run test:unit (163 passed)

Dependencies

• Platform and Dashboard: base44-dev/apper#13427
• Filesystem deployment: Deploy filesystem-backed user credential definitions cli#553