Implement friend APIs and target profile support

- Add friend search/list/request/requestCancel/response/expel handlers and schemas
- Store friend relationships in user_friend and backfill default friend links at startup
- Auto-add the default user as a friend for newly created accounts
- Support target user_id in /api profile requests and return target profile data
- Make profile accessory_info optional to avoid zero-value accessory payloads
- Update login topInfo friend counters from friend relationship state

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/startup/friend.go

@@ -0,0 +1,38 @@
+package startup
+
+import (
+	usermodel "honoka-chan/internal/model/user"
+	"honoka-chan/pkg/db"
+	"log"
+)
+
+func EnsureDefaultFriends() {
+	session := db.UserEng.NewSession()
+	defer session.Close()
+
+	if err := session.Begin(); err != nil {
+		log.Fatalln("初始化默认好友失败:", err.Error())
+	}
+
+	userIDs := []int{}
+	err := session.Table(new(usermodel.Users)).
+		Cols("user_id").
+		Find(&userIDs)
+	if err != nil {
+		session.Rollback()
+		log.Fatalln("初始化默认好友失败:", err.Error())
+	}
+
+	for _, userID := range userIDs {
+		err = usermodel.EnsureDefaultFriendship(session, userID)
+		if err != nil {
+			session.Rollback()
+			log.Fatalln("初始化默认好友失败:", err.Error())
+		}
+	}
+
+	if err := session.Commit(); err != nil {
+		session.Rollback()
+		log.Fatalln("初始化默认好友失败:", err.Error())
+	}
+}