feat(supply-api): 完成核心模块实现

新增/修改内容：
- config: 添加配置管理（config.example.yaml, config.go）
- cache: 添加Redis缓存层（redis.go）
- domain: 添加invariants不变量验证及测试
- middleware: 添加auth认证和idempotency幂等性中间件及测试
- repository: 添加完整数据访问层（account, package, settlement, idempotency, db）
- sql: 添加幂等性表DDL脚本

代码覆盖：
- auth middleware实现凭证边界验证
- idempotency middleware实现请求幂等性
- invariants实现业务不变量检查
- repository层实现完整的数据访问逻辑

关联issue: Round-1 R1-ISSUE-006 凭证边界硬门禁

supply-api/internal/repository/db.go

@@ -0,0 +1,81 @@
+package repository
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+	"lijiaoqiao/supply-api/internal/config"
+)
+
+// DB 数据库连接池
+type DB struct {
+	Pool *pgxpool.Pool
+}
+
+// NewDB 创建数据库连接池
+func NewDB(ctx context.Context, cfg config.DatabaseConfig) (*DB, error) {
+	poolConfig, err := pgxpool.ParseConfig(cfg.DSN())
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse database config: %w", err)
+	}
+
+	poolConfig.MaxConns = int32(cfg.MaxOpenConns)
+	poolConfig.MinConns = int32(cfg.MaxIdleConns)
+	poolConfig.MaxConnLifetime = cfg.ConnMaxLifetime
+	poolConfig.MaxConnIdleTime = cfg.ConnMaxIdleTime
+	poolConfig.HealthCheckPeriod = 30 * time.Second
+
+	pool, err := pgxpool.NewWithConfig(ctx, poolConfig)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create connection pool: %w", err)
+	}
+
+	// 验证连接
+	if err := pool.Ping(ctx); err != nil {
+		pool.Close()
+		return nil, fmt.Errorf("failed to ping database: %w", err)
+	}
+
+	return &DB{Pool: pool}, nil
+}
+
+// Close 关闭连接池
+func (db *DB) Close() {
+	if db.Pool != nil {
+		db.Pool.Close()
+	}
+}
+
+// HealthCheck 健康检查
+func (db *DB) HealthCheck(ctx context.Context) error {
+	return db.Pool.Ping(ctx)
+}
+
+// BeginTx 开始事务
+func (db *DB) BeginTx(ctx context.Context) (Transaction, error) {
+	tx, err := db.Pool.Begin(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return &txWrapper{tx: tx}, nil
+}
+
+// Transaction 事务接口
+type Transaction interface {
+	Commit(ctx context.Context) error
+	Rollback(ctx context.Context) error
+}
+
+type txWrapper struct {
+	tx pgxpool.Tx
+}
+
+func (t *txWrapper) Commit(ctx context.Context) error {
+	return t.tx.Commit(ctx)
+}
+
+func (t *txWrapper) Rollback(ctx context.Context) error {
+	return t.tx.Rollback(ctx)
+}