Refactor authentication system: centralize credential handling in Builder

Major architectural improvement to eliminate duplicate credential embedding
and simplify authentication logic across the codebase.

Changes:
- Builder now generates all URL variants with proper authentication upfront
  * RTSP: generates URLs with embedded credentials and without (for open cameras)
  * HTTP/JPEG/MJPEG: generates variants with query params and for Basic Auth
- Tester simplified to only test ready-to-use URLs without modification
  * Removed auth chain logic and multiple authentication method attempts
  * Removed AuthMethod enum and related complexity
  * Credentials automatically extracted from URLs when needed
- Scanner cleaned up by removing embedCredentialsInURL function
  * All TestStream calls now use single URL parameter
  * Removed AuthMethod from DiscoveredStream model

Benefits:
- Eliminates bug where credentials were added up to 3 times
- Centralizes all URL generation logic in one place (Builder)
- Cleaner, more maintainable code with clear separation of concerns
- Reduces complexity by ~200 lines of code
- All authentication scenarios still fully supported

internal/models/camera.go

@@ -67,7 +67,6 @@ type DiscoveredStream struct {
 	Protocol   string                 `json:"protocol"`
 	Port       int                    `json:"port"`
 	Working    bool                   `json:"working"`
-	AuthMethod string                 `json:"auth_method,omitempty"` // no_auth, basic_auth, query_params, combined, digest
 	Resolution string                 `json:"resolution,omitempty"`
 	Codec      string                 `json:"codec,omitempty"`
 	FPS        int                    `json:"fps,omitempty"`