refactor(signserver): replace raw SQL with repository interfaces

Extract all direct database access into three repository interfaces
(SignUserRepo, SignCharacterRepo, SignSessionRepo) matching the
pattern established in channelserver. This surfaces 9 previously
silenced errors that are now logged with structured context, and
makes the sign server testable with mock repos instead of go-sqlmock.

Security fix: GetFriends now uses parameterized ANY($1) queries
instead of string-concatenated WHERE clauses (SQL injection vector).

server/signserver/dsgn_resp.go

@@ -333,8 +333,10 @@ func (s *Session) makeSignResponse(uid uint32) []byte {
 	bf.WriteBytes(filters.Data())
 
 	if s.client == VITA || s.client == PS3 || s.client == PS4 {
-		var psnUser string
-		_ = s.server.db.QueryRow("SELECT psn_id FROM users WHERE id = $1", uid).Scan(&psnUser)
+		psnUser, err := s.server.userRepo.GetPSNIDForUser(uid)
+		if err != nil {
+			s.logger.Warn("Failed to get PSN ID for user", zap.Uint32("uid", uid), zap.Error(err))
+		}
 		bf.WriteBytes(stringsupport.PaddedString(psnUser, 20, true))
 	}