feat(usage-log): 增加请求 User-Agent 记录

在使用记录中添加 user_agent 字段，用于记录 API 请求的 User-Agent 头信息，
便于分析客户端类型和调试。

变更内容：
- 新增数据库迁移 028_add_usage_logs_user_agent.sql
- 更新 UsageLog 模型和 Ent Schema 添加 user_agent 字段
- 更新 Repository 层的 Create 和 scanUsageLog 方法
- 更新 RecordUsageInput 结构体支持传入 UserAgent
- 更新 Claude/OpenAI/Gemini 三个网关 Handler 传递 UserAgent

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/ent/usagelog/where.go

@@ -175,6 +175,11 @@ func FirstTokenMs(v int) predicate.UsageLog {
 	return predicate.UsageLog(sql.FieldEQ(FieldFirstTokenMs, v))
 }
 
+// UserAgent applies equality check predicate on the "user_agent" field. It's identical to UserAgentEQ.
+func UserAgent(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldEQ(FieldUserAgent, v))
+}
+
 // ImageCount applies equality check predicate on the "image_count" field. It's identical to ImageCountEQ.
 func ImageCount(v int) predicate.UsageLog {
 	return predicate.UsageLog(sql.FieldEQ(FieldImageCount, v))
@@ -1110,6 +1115,81 @@ func FirstTokenMsNotNil() predicate.UsageLog {
 	return predicate.UsageLog(sql.FieldNotNull(FieldFirstTokenMs))
 }
 
+// UserAgentEQ applies the EQ predicate on the "user_agent" field.
+func UserAgentEQ(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldEQ(FieldUserAgent, v))
+}
+
+// UserAgentNEQ applies the NEQ predicate on the "user_agent" field.
+func UserAgentNEQ(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldNEQ(FieldUserAgent, v))
+}
+
+// UserAgentIn applies the In predicate on the "user_agent" field.
+func UserAgentIn(vs ...string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldIn(FieldUserAgent, vs...))
+}
+
+// UserAgentNotIn applies the NotIn predicate on the "user_agent" field.
+func UserAgentNotIn(vs ...string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldNotIn(FieldUserAgent, vs...))
+}
+
+// UserAgentGT applies the GT predicate on the "user_agent" field.
+func UserAgentGT(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldGT(FieldUserAgent, v))
+}
+
+// UserAgentGTE applies the GTE predicate on the "user_agent" field.
+func UserAgentGTE(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldGTE(FieldUserAgent, v))
+}
+
+// UserAgentLT applies the LT predicate on the "user_agent" field.
+func UserAgentLT(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldLT(FieldUserAgent, v))
+}
+
+// UserAgentLTE applies the LTE predicate on the "user_agent" field.
+func UserAgentLTE(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldLTE(FieldUserAgent, v))
+}
+
+// UserAgentContains applies the Contains predicate on the "user_agent" field.
+func UserAgentContains(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldContains(FieldUserAgent, v))
+}
+
+// UserAgentHasPrefix applies the HasPrefix predicate on the "user_agent" field.
+func UserAgentHasPrefix(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldHasPrefix(FieldUserAgent, v))
+}
+
+// UserAgentHasSuffix applies the HasSuffix predicate on the "user_agent" field.
+func UserAgentHasSuffix(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldHasSuffix(FieldUserAgent, v))
+}
+
+// UserAgentIsNil applies the IsNil predicate on the "user_agent" field.
+func UserAgentIsNil() predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldIsNull(FieldUserAgent))
+}
+
+// UserAgentNotNil applies the NotNil predicate on the "user_agent" field.
+func UserAgentNotNil() predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldNotNull(FieldUserAgent))
+}
+
+// UserAgentEqualFold applies the EqualFold predicate on the "user_agent" field.
+func UserAgentEqualFold(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldEqualFold(FieldUserAgent, v))
+}
+
+// UserAgentContainsFold applies the ContainsFold predicate on the "user_agent" field.
+func UserAgentContainsFold(v string) predicate.UsageLog {
+	return predicate.UsageLog(sql.FieldContainsFold(FieldUserAgent, v))
+}
+
 // ImageCountEQ applies the EQ predicate on the "image_count" field.
 func ImageCountEQ(v int) predicate.UsageLog {
 	return predicate.UsageLog(sql.FieldEQ(FieldImageCount, v))