feat(returns): canonical consolidation for returns/refunds (supersedes #14, #18)

[rmak78]

Summary

This PR creates the canonical returns/refunds consolidation branch and supersedes overlapping work in #14 and #18.

What this keeps

• Reason-code support for returns (return_reason_codes, sales_returns.return_reason_code)
• Approval workflow fields on returns (approval_status, approved_by_employee_id, approved_at)
• Refund method tracking + status indexing on sales_refunds
• Existing inventory reversal behavior remains unchanged in API flow (sales-return-lines restock path)

Implementation

• Adds one migration: data-model/migrations/0011_returns_refunds_controls.sql
• Uses ALTER TABLE on existing baseline returns/refunds tables (no duplicate replacement tables)
• Adds indexes for approval/reason/status query paths

Validation

• npx wrangler deploy --dry-run ✅

Supersedes

• feat(finops): sales returns/refunds with inventory reversal baseline #14
• feat(returns): add controlled returns/refunds schema for approvals and refund methods #18

Please close #14 and #18 after this PR is accepted as the canonical path.