security: harden against SAST findings (criticals through mediums)

Critical:
- Validate baseURL/sourceURL/upstreamURL at config load time
  (prevents XML injection, XSS, SSRF via config/env manipulation)
- Use xml.Escape for OpenSearch XML template interpolation

High:
- Add security headers middleware (CSP, X-Frame-Options, HSTS, etc.)
- Sanitize result URLs to reject javascript:/data: schemes
- Sanitize infobox img_src against dangerous URL schemes
- Default CORS to deny-all (was wildcard *)

Medium:
- Rate limiter: X-Forwarded-For only trusted from configured proxies
- Validate engine names against known registry allowlist
- Add 1024-char max query length
- Sanitize upstream error messages (strip raw response bodies)
- Upstream client validates URL scheme (http/https only)

Test updates:
- Update extractIP tests for new trusted proxy behavior

internal/engines/youtube.go

@@ -78,7 +78,7 @@ func (e *YouTubeEngine) Search(ctx context.Context, req contracts.SearchRequest)
 
 	if resp.StatusCode != http.StatusOK {
 		body, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
-		return contracts.SearchResponse{}, fmt.Errorf("youtube api error: status=%d body=%q", resp.StatusCode, string(body))
+		return contracts.SearchResponse{}, fmt.Errorf("youtube api error: status %d", resp.StatusCode)
 	}
 
 	var apiResp youtubeSearchResponse
@@ -87,7 +87,7 @@ func (e *YouTubeEngine) Search(ctx context.Context, req contracts.SearchRequest)
 	}
 
 	if apiResp.Error != nil {
-		return contracts.SearchResponse{}, fmt.Errorf("youtube api error: %s", apiResp.Error.Message)
+		return contracts.SearchResponse{}, fmt.Errorf("youtube api error: code %d", apiResp.Error.Code)
 	}
 
 	results := make([]contracts.MainResult, 0, len(apiResp.Items))