feat: migration script for USDC grants to ACT grants

[baktun14]

Context

Part of AEP-76 BME migration. After the chain upgrade enables BME and the funding wallet receives pre-minted ACT, existing managed wallet users need their USDC authz grants converted to ACT authz grants.

Tasks

• Create apps/api/src/billing/scripts/migrate-to-bme.ts
• Script should:
  1. Query all user wallets with non-zero deploymentAllowance
  2. For each wallet:
     • Read current USDC authz DepositAuthorization grant amount
     • Revoke USDC DepositAuthorization grant
     • Create new ACT DepositAuthorization grant with equivalent amount
     • Update deploymentAllowance in DB
  3. Verify fee allowance grants (uakt) are still valid
  4. Log progress and handle errors gracefully
  5. Support dry-run mode for testing

Cutover Sequence

Step 1: DEPLOYMENT_GRANT_DENOM=uusdc                    ← current state
Step 2: Chain upgrade (BME enabled, funding wallet gets ACT)
Step 3: Run this migration script
Step 4: DEPLOYMENT_GRANT_DENOM=uact, NEXT_PUBLIC_MANAGED_WALLET_DENOM=uact

Acceptance Criteria

• All managed wallets have valid ACT authz grants after migration
• USDC authz grants are revoked
• DB deploymentAllowance values match new ACT grant amounts
• Fee allowance grants (uakt) are preserved
• Dry-run mode works for testing on sandbox/testnet
• Script is idempotent (safe to re-run)

Milestone

AEP-76: BME Migration — Phase 7 (Migration)

Reference

• docs/aep-76-bme-migration-plan.md