feat(backend-sep12): support customer deletion requests

[GideonBature]

Summary

• add authenticated SEP-12 customer deletion support for stored KYC metadata
• enforce authenticated account ownership for SEP-12 customer PUT and DELETE operations
• add controller coverage for successful deletion, missing customer metadata, and account mismatch handling

Validation

• npm test -w backend -- --runInBand --coverage=false src/api/controllers/sep12.controller.test.ts
• npx eslint backend/src/api/controllers/sep12.controller.ts backend/src/api/controllers/sep12.controller.test.ts backend/src/api/routes/sep12.route.ts

Note: npm run test:backend currently fails on upstream-wide tests that require additional environment/services or unrelated fixes, including PostgreSQL DATABASE_URL, SIGNING_KEY, Redis-backed E2E flows, and existing migration/KYC provider/recurring payment tests.

Closes #617

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