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/handlers/imageenc.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 	"strconv"
@@ -57,7 +58,7 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		s.log.Debug("encrypted image request",
 			"host", req.SourceHost,
 			"path", req.SourcePath,
-			"size", req.Size,
+			"dimensions", fmt.Sprintf("%dx%d", req.Size.Width, req.Size.Height),
 			"format", req.Format,
 		)