fix(api): use query param instead of body for GET /backend/monitor

[Bahtya]

Problem

The /backend/monitor endpoint uses HTTP GET but its Swagger annotation declares a request body (@Param request body schema.BackendMonitorRequest). This violates REST conventions and breaks:

• OpenAPI code generators
• Swagger UI (confusing UX)
• Standard HTTP clients that strip body from GET requests

Root Cause

The Swagger annotation in backend_monitor.go uses @Param request body for a GET endpoint. Per OpenAPI spec, GET requests should not have request bodies.

Solution

Changed the Swagger annotation from body param to query param:

// Before:
// @Param request body schema.BackendMonitorRequest true "Backend statistics request"

// After:
// @Param model query string true "Model name"

Echo's c.Bind() already handles query parameters for GET requests, so no code changes are needed — only the annotation fix.

Testing

• The endpoint already works with query params via Echo's Bind() — this just fixes the documentation/codegen.

Fixes #9207