refactor(channelserver): introduce repository interfaces for all 21 repos

Replace concrete pointer types on the Server struct with interfaces to
decouple handlers from PostgreSQL implementations. This enables mock/stub
injection for unit tests and opens the door to alternative storage
backends (SQLite, in-memory).

Also adds 9 missing repo initializations to SetTestDB() (event,
achievement, shop, cafe, goocoo, diva, misc, scenario, mercenary)
to match NewServer().

server/channelserver/testhelpers_db.go

@@ -395,4 +395,13 @@ func SetTestDB(s *Server, db *sqlx.DB) {
 	s.stampRepo = NewStampRepository(db)
 	s.distRepo = NewDistributionRepository(db)
 	s.sessionRepo = NewSessionRepository(db)
+	s.eventRepo = NewEventRepository(db)
+	s.achievementRepo = NewAchievementRepository(db)
+	s.shopRepo = NewShopRepository(db)
+	s.cafeRepo = NewCafeRepository(db)
+	s.goocooRepo = NewGoocooRepository(db)
+	s.divaRepo = NewDivaRepository(db)
+	s.miscRepo = NewMiscRepository(db)
+	s.scenarioRepo = NewScenarioRepository(db)
+	s.mercenaryRepo = NewMercenaryRepository(db)
 }