Fix validation and withdrawal verification

[bidexbusuyi-eng]

Summary

• Validate signup/login emails and withdrawal bank details before processing
• Require Paystack account resolution before any payout is attempted
• Update the OpenAPI contract to document the 422 failure path
• Add a CI lint check for the spec and Paystack response deserialization unit coverage

Closes #912
Closes #913
Closes #914
Closes #915

Notes

• The fix is kept isolated to the working branch created from the forked repository.
• No unrelated workspace edits or lockfile changes were included in the PR.

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