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/database.go

@@ -27,6 +27,7 @@ func CreateTables() {
 	db.UserEng.Sync2(new(usermodel.UserLiveGoal))
 	db.UserEng.Sync2(new(usermodel.UserLiveInProgress))
 	db.UserEng.Sync2(new(usermodel.UserLiveRecord))
+	db.UserEng.Sync2(new(usermodel.UserFriend))
 	db.UserEng.Sync2(new(usermodel.UserPref))
 	db.UserEng.Sync2(new(usermodel.Users))
 	db.UserEng.Sync2(new(usermodel.UserUnit))