fix(middleware): Correct rate limit header type conversion

- Fix string(rune(remaining)) to strconv.Itoa(remaining) in device_auth.go
- Prevents garbage characters in X-RateLimit-Remaining header
- No functionality changes, only fixes broken headers

Testing: Verified with code inspection that headers return proper integers

internal/middleware/device_auth.go

@@ -88,7 +88,7 @@ func (m *DeviceAuthMiddleware) Authenticate(next echo.HandlerFunc) echo.HandlerF
 
 		remaining := m.rateLimiter.GetRemainingRequests(deviceID, requestType, config)
 		c.Response().Header().Set("X-RateLimit-Limit", "60")
-		c.Response().Header().Set("X-RateLimit-Remaining", string(rune(remaining)))
+		c.Response().Header().Set("X-RateLimit-Remaining", strconv.Itoa(remaining))
 
 		ctx := DeviceContext{
 			ID:               device.ID.Bytes,