Add upstream connection info and download metrics to logging

- Capture TLS version, cipher suite, HTTP version, and remote addr
- Add download bitrate using go-humanize SI formatting
- Use consistent WxH format for dimensions (not struct notation)
- Rename input/output to src/dst for consistency
- Add separate "upstream fetched" log with connection details

internal/imgcache/service.go

@@ -9,6 +9,8 @@ import (
 	"log/slog"
 	"net/url"
 	"time"
+
+	"github.com/dustin/go-humanize"
 )
 
 // Service implements the ImageCache interface, orchestrating cache, fetcher, and processor.
@@ -144,6 +146,29 @@ func (s *Service) fetchAndProcess(ctx context.Context, req *ImageRequest) (*Imag
 		return nil, fmt.Errorf("failed to read upstream response: %w", err)
 	}
 
+	// Calculate download bitrate
+	fetchBytes := int64(len(sourceData))
+	var downloadRate string
+
+	if fetchResult.FetchDurationMs > 0 {
+		seconds := float64(fetchResult.FetchDurationMs) / 1000.0 //nolint:mnd // ms to seconds
+		bitsPerSecond := float64(fetchBytes*8) / seconds         //nolint:mnd // bytes to bits
+		downloadRate = humanize.SI(bitsPerSecond, "bps")
+	}
+
+	// Log upstream fetch details
+	s.log.Info("upstream fetched",
+		"host", req.SourceHost,
+		"path", req.SourcePath,
+		"bytes", fetchBytes,
+		"fetch_ms", fetchResult.FetchDurationMs,
+		"rate", downloadRate,
+		"remote_addr", fetchResult.RemoteAddr,
+		"http", fetchResult.HTTPVersion,
+		"tls", fetchResult.TLSVersion,
+		"cipher", fetchResult.TLSCipherSuite,
+	)
+
 	// Validate magic bytes match content type
 	if err := ValidateMagicBytes(sourceData, fetchResult.ContentType); err != nil {
 		return nil, fmt.Errorf("content validation failed: %w", err)
@@ -167,19 +192,18 @@ func (s *Service) fetchAndProcess(ctx context.Context, req *ImageRequest) (*Imag
 	processDuration := time.Since(processStart)
 
 	// Log conversion details
-	inputSize := int64(len(sourceData))
 	outputSize := processResult.ContentLength
-	sizePercent := float64(outputSize) / float64(inputSize) * 100.0 //nolint:mnd // percentage calculation
+	sizePercent := float64(outputSize) / float64(fetchBytes) * 100.0 //nolint:mnd // percentage calculation
 
 	s.log.Info("image converted",
 		"host", req.SourceHost,
 		"path", req.SourcePath,
-		"input_format", processResult.InputFormat,
-		"output_format", req.Format,
-		"input_bytes", inputSize,
-		"output_bytes", outputSize,
-		"input_dimensions", fmt.Sprintf("%dx%d", processResult.InputWidth, processResult.InputHeight),
-		"output_dimensions", fmt.Sprintf("%dx%d", processResult.Width, processResult.Height),
+		"src_format", processResult.InputFormat,
+		"dst_format", req.Format,
+		"src_bytes", fetchBytes,
+		"dst_bytes", outputSize,
+		"src_dimensions", fmt.Sprintf("%dx%d", processResult.InputWidth, processResult.InputHeight),
+		"dst_dimensions", fmt.Sprintf("%dx%d", processResult.Width, processResult.Height),
 		"size_ratio", fmt.Sprintf("%.1f%%", sizePercent),
 		"convert_ms", processDuration.Milliseconds(),
 		"quality", req.Quality,