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/api/handlers/discover.go

@@ -103,7 +103,7 @@ func (h *DiscoverHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	result, err := h.scanner.Scan(r.Context(), req, streamWriter)
 	if err != nil {
 		h.logger.Error("discovery failed", err)
-		streamWriter.SendError(err)
+		_ = streamWriter.SendError(err)
 		return
 	}
 
@@ -126,5 +126,5 @@ func (h *DiscoverHandler) sendErrorResponse(w http.ResponseWriter, message strin
 		"code":    statusCode,
 	}
 
-	json.NewEncoder(w).Encode(response)
+	_ = json.NewEncoder(w).Encode(response)
 }