feat: implement unlink connected bank account API

[AdityaK9822]

Closes #488

Description

This PR implements the API endpoint to safely unlink a connected fiat bank account from an
employee's profile within an organization.

Instead of a hard deletion, this implementation uses a "soft-delete" approach by tracking when
a bank account was unlinked, ensuring that historical transaction references and audit trails
are preserved.

Changes

• Database: Added bankAccountDeletedAt timestamp to the employees table in
  src/server/db/schema.ts.
• Service Layer:
  • Implemented unlinkBankAccount in BankAccountService to mark accounts as unlinked.
  • Updated getEmployeeAccount to filter out accounts that have been unlinked.
• API: Created a new DELETE endpoint at /api/v1/finance/accounts/[id] that verifies the
  authenticated user's organization ownership before unlinking the account.

Verification

• Verified that DELETE requests for employees outside the user's organization are rejected
  with a 403.
• Verified that unlinked accounts are no longer returned by the account retrieval API.
• Confirmed that the process preserves the employee record and does not impact the
  fiatTransactions table.

[vercel]

@AdityaK9822 is attempting to deploy a commit to the vestroll's projects Team on Vercel.

A member of the Team first needs to authorize it.

[codeZe-us]

@AdityaK9822 PR should point to dev branch not main branch

[AdityaK9822]

Here, I have changed the base into dev branch