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/schema/api/api.go

@@ -2,6 +2,7 @@ package apischema
 
 type ApiReq struct {
 	Module    string `json:"module"`
+	UserID    int    `json:"user_id"`
 	Action    string `json:"action"`
 	Timestamp int64  `json:"timeStamp"`
 }