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/imgcache.go

@@ -184,6 +184,12 @@ type FetchResult struct {
 	FetchDurationMs int64
 	// RemoteAddr is the IP:port of the upstream server
 	RemoteAddr string
+	// HTTPVersion is the protocol version (e.g., "1.1", "2.0")
+	HTTPVersion string
+	// TLSVersion is the TLS protocol version (e.g., "TLS 1.3")
+	TLSVersion string
+	// TLSCipherSuite is the negotiated cipher suite name
+	TLSCipherSuite string
 }
 
 // Processor handles image transformation (resize, format conversion)