fix(bridge): add external account webhook config

[patoo0x]

Summary

• Adds the missing Bridge external_account webhook public-key placeholder to dev/config/base-config.yaml.
• Regenerates public schema and supergraph so the external-account link payload matches the current GraphQL code.

Test Plan

• npx prettier --check dev/config/base-config.yaml
• yarn check:sdl

Notes

• This is intentionally separate from ENG-406 and ENG-274.
• Without this, latest tmp/bridge-rebase-pr-ready fails config validation because bridge.webhook.publicKeys.external_account is required by the schema but absent from the base config.

[patoo0x]

Companion to ENG-406 PR #389: this base-branch config/schema fix removes the temporary shim needed to run #389's GraphQL mutation test and check:sdl cleanly.