refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/internal/store/db.go

@@ -0,0 +1,70 @@
+package store
+
+import (
+	"fmt"
+	"log"
+	"opencatd-open/pkg/config"
+
+	_ "github.com/lib/pq"
+	"gorm.io/driver/mysql"
+	"gorm.io/driver/postgres"
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+)
+
+var DB *gorm.DB
+
+func InitDB(cfg *config.Config) (*gorm.DB, error) {
+	var dialector gorm.Dialector
+
+	switch cfg.DB_Type {
+	case "sqlite":
+		dialector = sqliteDialector(cfg.DSN)
+	case "postgres":
+		dialector = postgresDialector(cfg.DSN)
+	case "mysql":
+		dialector = mysqlDialector(cfg.DSN)
+	default:
+		return nil, fmt.Errorf("unsupported database type: %s", cfg.DB_Type)
+	}
+
+	db, err := gorm.Open(dialector, &gorm.Config{})
+	if err != nil {
+		return nil, fmt.Errorf("failed to connect database: %w", err)
+	}
+
+	sqlDB, err := db.DB()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get underlying *sql.DB: %w", err)
+	}
+	sqlDB.SetMaxOpenConns(cfg.DBMaxOpenConns)
+	sqlDB.SetMaxIdleConns(cfg.DBMaxIdleConns)
+
+	if err := db.AutoMigrate(AllModels()...); err != nil {
+		log.Printf("AutoMigrate warning: %v", err)
+	}
+
+	DB = db
+	return db, nil
+}
+
+func sqliteDialector(dsn string) gorm.Dialector {
+	if dsn == "" {
+		dsn = "opencatd.db"
+	}
+	return sqlite.Open(dsn)
+}
+
+func postgresDialector(dsn string) gorm.Dialector {
+	if dsn == "" {
+		dsn = "host=localhost user=postgres password=postgres dbname=opencatd port=5432 sslmode=disable"
+	}
+	return postgres.Open(dsn)
+}
+
+func mysqlDialector(dsn string) gorm.Dialector {
+	if dsn == "" {
+		dsn = "root:password@tcp(127.0.0.1:3306)/opencatd?charset=utf8mb4&parseTime=True&loc=Local"
+	}
+	return mysql.Open(dsn)
+}