feat: Swagger addition/OpenAPI documentation for all endpoints

[EmmyKay0026]

Closes #405

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Add Swagger/OpenAPI documentation for backend APIs

This PR adds comprehensive Swagger/OpenAPI documentation for the backend API, including controller tags, operation summaries, request/response metadata, DTO property descriptions, and JWT bearer auth support in the generated docs.

What changed

• Configured Swagger UI and OpenAPI generation at /api/docs
• Added bearer auth support for protected endpoints
• Documented core and secondary backend controllers, including auth, escrow, admin, notifications, webhook, app version,
• API key, IPFS, and Stellar endpoints
• Added DTO-level property metadata for request payloads
• Added a regression test to ensure Swagger decorators remain covered
• Added CI enforcement for Swagger documentation coverage

Verification

• Ran the Swagger regression test: passed
• Ran the backend build: succeeded

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