fix: increase SSE scanner max line size from 40MB to 500MB

4K image base64 data can exceed 40MB limit, causing "bufio.Scanner:
token too long" errors. Scanner is adaptive (starts at 64KB, grows
as needed), so increasing the cap has no impact on normal responses.

backend/internal/service/gateway_service.go

@@ -41,7 +41,7 @@ const (
 	claudeAPIURL            = "https://api.anthropic.com/v1/messages?beta=true"
 	claudeAPICountTokensURL = "https://api.anthropic.com/v1/messages/count_tokens?beta=true"
 	stickySessionTTL        = time.Hour // 粘性会话TTL
-	defaultMaxLineSize      = 40 * 1024 * 1024
+	defaultMaxLineSize      = 500 * 1024 * 1024
 	// Canonical Claude Code banner. Keep it EXACT (no trailing whitespace/newlines)
 	// to match real Claude CLI traffic as closely as possible. When we need a visual
 	// separator between system blocks, we add "\n\n" at concatenation time.