fix: resolve P0/P1 code quality issues

P0 fixes:
- ModelError.Is(): use exact matching instead of substring contains()
- shouldClearStickySession: add context param for cancellation/tracing

P1 fixes:
- TODO stubs: return 501 Not Implemented errors
- validateInstanceSignature: deduplicate to shared validateCodeSignature()
- Error messages: standardize to English only
- http.go: remove pseudo if-else with duplicate branches

backend/migrations/029_usage_log_image_fields.sql

@@ -0,0 +1,5 @@
+-- 为使用日志添加图片生成统计字段
+-- 用于记录 gemini-3-pro-image 等图片生成模型的使用情况
+
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS image_count INT DEFAULT 0;
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS image_size VARCHAR(10);