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/handler/handler.go

@@ -8,6 +8,7 @@ import (
 	_ "honoka-chan/internal/handler/background"
 	_ "honoka-chan/internal/handler/download"
 	_ "honoka-chan/internal/handler/event"
+	_ "honoka-chan/internal/handler/friend"
 	_ "honoka-chan/internal/handler/gdpr"
 	_ "honoka-chan/internal/handler/ghome"
 	_ "honoka-chan/internal/handler/lbonus"