feat(backend): Add Swagger/OpenAPI documentation for all endpoints

[KuchiMercy]

Description

@nestjs/swagger and swagger-ui-express installed but API docs incomplete. Add comprehensive auto-generated docs.

Acceptance Criteria

• @apitags() on all controllers (Escrow, Auth, Admin, Assets, Health, Webhooks, Notifications)
• @apioperation() with summary on every handler
• @apiresponse() for all response codes (200, 201, 400, 401, 403, 404, 409, 429, 500)
• @ApiBearerAuth() on authenticated endpoints
• @ApiBody() with examples for POST/PATCH/PUT
• @apiquery() for query parameters
• @ApiParam() for path parameters
• @ApiProperty() on all DTOs with descriptions
• Configure Swagger UI at /api/docs with JWT bearer input
• Example request/response for core escrow CRUD
• WebSocket events documentation
• CI check for missing Swagger decorators

Points: 150

[bheesty]

@bheesty has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, I can fix this issue, Kindly assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @bheesty to this issue.

[drips-wave]

Congratulations, @EmmyKay0026! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @EmmyKay0026: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

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