Stripe Embedded Onboarding for new sellers from Canada

[ershad]

Originally built by @hchhabra.

What

Replace the custom payments onboarding flow with Stripe's Embedded Onboarding for new Canadian sellers. When enabled via the stripe_embedded_onboarding feature flag, new sellers from Canada who
don't yet have a Stripe account see Stripe's ConnectAccountOnboarding component instead of the custom bank account form and verification section.

This is Phase 1 of #4021 — scoped to new Canadian sellers only, with the plan to expand to other countries and existing sellers in later phases.

Changes:

• Add StripeEmbeddedOnboarding React component wrapping Stripe's ConnectAccountOnboarding with Gumroad's ToS/privacy URLs and eventually_due collection
• Extend StripeAccountSessionsController to support account_onboarding component type, creating a minimal Stripe custom account on first request
• Sync bank accounts and debit cards from Stripe to local records via account.updated webhook (sync_external_accounts_from_stripe)
• Gate on feature flag + Canada-only + no existing Stripe account
• Hide Verification section when embedded onboarding is active
• Mark merchant accounts created through this flow with stripe_embedded_onboarding: true to distinguish onboarding paths
• Skip HandleNewBankAccountWorker for bank accounts synced from Stripe (already have a stripe_bank_account_id)

Why

The custom onboarding flow maintains 100+ country-specific bank account models, custom compliance forms, and complex validation logic. Stripe's embedded component handles identity verification,
bank account collection, and tax info in one step — reducing support tickets and maintenance burden. Starting with Canada to validate the approach before expanding.

Test results

Tests added for:

• Account session creation with account_onboarding component
• Stripe account creation, reuse, and resume for unverified accounts
• Bank account and debit card syncing from webhook
• External account replacement and deletion
• show_stripe_embedded_onboarding? gating logic (feature flag, country, account state)

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