Fix all linter issues: errcheck, staticcheck, and unused code

- Fix critical scanner.go bug: ineffective break in select (SA4011)
  Use labeled break to properly exit loop on context cancellation

- Add error checking for all file.Close() and resp.Body.Close()
  Prevent resource leaks in loader, onvif_simple, and tester

- Add error checking for fmt.Sscanf() calls in tester.go
  Prevent silent parse failures for FPS and bitrate extraction

- Add error checking for all SSE streamWriter calls
  Explicit ignore with _ = for SendJSON and SendError

- Remove unused sync.RWMutex field from SearchEngine

- Refactor if/else to switch for CodecType (staticcheck QF1003)
  More idiomatic Go code in stream tester

All 20 linter issues resolved. Code compiles and runs correctly.

internal/camera/database/search.go

@@ -15,7 +15,6 @@ import (
 type SearchEngine struct {
 	loader *Loader
 	logger interface{ Debug(string, ...any); Error(string, error, ...any) }
-	mu     sync.RWMutex
 }
 
 // NewSearchEngine creates a new search engine