docs: explain Python account migration

[viezhukai-stack]

Summary

• explain which credentials can be migrated from the Python version
• document the supported TXT and JSON import formats in both READMEs
• clarify that legacy pool metadata is not migrated and is resynced by the Go version
• document the current import limits and the Grok Web UI path

This addresses the migration question in #736.

Verification

• git diff --check
• verified the documented TXT/JSON shapes against backend/internal/infra/provider/web/import.go
• verified the legacy pool JSON rejection against backend/internal/infra/provider/web/import_test.go
• verified the 1,000-file / 30 MiB limits against backend/internal/transport/http/account/handler.go
• verified the quick/file import actions against frontend/src/features/accounts/accounts-page.tsx
• ran README content and source-contract assertions locally

Documentation-only change; the local environment does not currently have the Go toolchain installed.