feat: complete swagger api documentation with auth examples

[fahatadam]

Closes #279

What was done

Added comprehensive Swagger API documentation to the NestJS backend. All module Controllers (Auth, Admin, Escrows, Assets, API Keys, Notifications, Webhooks, IPFS, Stellar Events) have been decorated with @ApiTags, @ApiOperation, @ApiBearerAuth, and standard error response descriptions. DTOs across these modules have been explicitly documented using @ApiProperty() and @ApiPropertyOptional() with descriptions and examples. The main.ts entry file was updated to properly document the custom Stellar Wallet signature challenge-response authentication mechanism.

Why it was done

The previously generated /api/docs endpoint was sparse and incomplete. Endpoints lacked proper Request/Response DTO reflection, required field indicators, and authentication requirements, making integration and frontend development difficult. The new decorators provide a fully interactive and descriptive OpenAPI specification.

How it was verified

• Verified code changes conform to standard @nestjs/swagger typescript types.
• Checked against all issue acceptance criteria.
  (Note: Please start the backend and verify the /api/docs endpoint rendering in your local environment.)

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