feat(error-handling): add global 404 JSON response filter

[lynaDev2]

Summary

Adds a global NestJS NotFound filter to return a consistent JSON response for unmatched routes.

Changes

Implemented NotFoundFilter

Registered filter in main.ts

Returns { statusCode: 404, message: "Route not found" }

Added test to verify behavior

Acceptance Criteria

Unknown routes return 404 with correct JSON structure

Existing routes continue to function normally

Tests pass successfully

How To Test

Start backend

Send GET /some/nonexistent/route

Expect 404 with { statusCode: 404, message: "Route not found" }