refactor(channelserver): extract FestaRepository and TowerRepository

Move all direct DB calls from handlers_festa.go (23 calls across 8
tables) and handlers_tower.go (16 calls across 4 tables) into
dedicated repository structs following the established pattern.

FestaRepository (14 methods): lifecycle cleanup, event management,
team souls, trial stats/rankings, user state, voting, registration,
soul submission, prize claiming/enumeration.

TowerRepository (12 methods): personal tower data (skills, progress,
gems), guild tenrouirai progress/scores/page advancement, tower RP.

Also fix pre-existing nil pointer panics in integration tests by
adding SetTestDB helper that initializes both the DB connection and
all repositories, and wire the done channel in createTestServerWithDB
to prevent Shutdown panics.

server/channelserver/testhelpers_db.go

@@ -329,3 +329,16 @@ func CreateTestGuild(t *testing.T, db *sqlx.DB, leaderCharID uint32, name string
 
 	return guildID
 }
+
+// SetTestDB assigns a database to a Server and initializes all repositories.
+// Use this in integration tests instead of setting s.server.db directly.
+func SetTestDB(s *Server, db *sqlx.DB) {
+	s.db = db
+	s.charRepo = NewCharacterRepository(db)
+	s.guildRepo = NewGuildRepository(db)
+	s.userRepo = NewUserRepository(db)
+	s.gachaRepo = NewGachaRepository(db)
+	s.houseRepo = NewHouseRepository(db)
+	s.festaRepo = NewFestaRepository(db)
+	s.towerRepo = NewTowerRepository(db)
+}