feat: add community enhancements — password reset, audit logs, multi-source backup

Three community-requested features:

1. CLI password reset: `backupx reset-password --username admin --password xxx`
   Docker users can run via `docker exec`. No full app init needed.

2. Audit logging: async fire-and-forget audit trail for all key operations
   (login, CRUD on tasks/targets/records, settings changes).
   New UI page at /audit with category filter and pagination.

3. Multi-source path backup: file backup tasks now support multiple source
   directories packed into a single tar archive. Backward compatible with
   existing single sourcePath field.

server/internal/service/database_discovery_service.go

@@ -0,0 +1,141 @@
+package service
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"strings"
+	"time"
+
+	"backupx/server/internal/apperror"
+	"backupx/server/internal/backup"
+)
+
+type DatabaseDiscoverInput struct {
+	Type     string `json:"type" binding:"required,oneof=mysql postgresql"`
+	Host     string `json:"host" binding:"required"`
+	Port     int    `json:"port" binding:"required,min=1"`
+	User     string `json:"user" binding:"required"`
+	Password string `json:"password" binding:"required"`
+}
+
+type DatabaseDiscoverResult struct {
+	Databases []string `json:"databases"`
+}
+
+type DatabaseDiscoveryService struct {
+	executor backup.CommandExecutor
+}
+
+func NewDatabaseDiscoveryService(executor backup.CommandExecutor) *DatabaseDiscoveryService {
+	return &DatabaseDiscoveryService{executor: executor}
+}
+
+func (s *DatabaseDiscoveryService) Discover(ctx context.Context, input DatabaseDiscoverInput) (*DatabaseDiscoverResult, error) {
+	switch strings.TrimSpace(strings.ToLower(input.Type)) {
+	case "mysql":
+		return s.discoverMySQL(ctx, input)
+	case "postgresql":
+		return s.discoverPostgreSQL(ctx, input)
+	default:
+		return nil, apperror.BadRequest("DATABASE_DISCOVER_INVALID_TYPE", "不支持的数据库类型", nil)
+	}
+}
+
+func (s *DatabaseDiscoveryService) discoverMySQL(ctx context.Context, input DatabaseDiscoverInput) (*DatabaseDiscoverResult, error) {
+	mysqlPath, err := s.executor.LookPath("mysql")
+	if err != nil {
+		return nil, apperror.BadRequest("DATABASE_DISCOVER_MYSQL_NOT_FOUND", "系统未安装 mysql 客户端", err)
+	}
+
+	timeout, cancel := context.WithTimeout(ctx, 5*time.Second)
+	defer cancel()
+
+	var stdout, stderr bytes.Buffer
+	args := []string{
+		fmt.Sprintf("--host=%s", input.Host),
+		fmt.Sprintf("--port=%d", input.Port),
+		fmt.Sprintf("--user=%s", input.User),
+		"-e", "SHOW DATABASES",
+		"--skip-column-names",
+	}
+	env := []string{fmt.Sprintf("MYSQL_PWD=%s", input.Password)}
+
+	if err := s.executor.Run(timeout, mysqlPath, args, backup.CommandOptions{
+		Stdout: &stdout,
+		Stderr: &stderr,
+		Env:    env,
+	}); err != nil {
+		errMsg := strings.TrimSpace(stderr.String())
+		if errMsg == "" {
+			errMsg = err.Error()
+		}
+		return nil, apperror.BadRequest("DATABASE_DISCOVER_MYSQL_FAILED", fmt.Sprintf("连接 MySQL 失败：%s", sanitizeMessage(errMsg)), err)
+	}
+
+	systemDBs := map[string]bool{
+		"information_schema": true,
+		"performance_schema": true,
+		"mysql":              true,
+		"sys":                true,
+	}
+
+	var databases []string
+	for _, line := range strings.Split(stdout.String(), "\n") {
+		db := strings.TrimSpace(line)
+		if db == "" || systemDBs[db] {
+			continue
+		}
+		databases = append(databases, db)
+	}
+
+	return &DatabaseDiscoverResult{Databases: databases}, nil
+}
+
+func (s *DatabaseDiscoveryService) discoverPostgreSQL(ctx context.Context, input DatabaseDiscoverInput) (*DatabaseDiscoverResult, error) {
+	psqlPath, err := s.executor.LookPath("psql")
+	if err != nil {
+		return nil, apperror.BadRequest("DATABASE_DISCOVER_PSQL_NOT_FOUND", "系统未安装 psql 客户端", err)
+	}
+
+	timeout, cancel := context.WithTimeout(ctx, 5*time.Second)
+	defer cancel()
+
+	var stdout, stderr bytes.Buffer
+	args := []string{
+		"-h", input.Host,
+		"-p", fmt.Sprintf("%d", input.Port),
+		"-U", input.User,
+		"-d", "postgres",
+		"-t", "-A",
+		"-c", "SELECT datname FROM pg_database WHERE datistemplate = false ORDER BY datname",
+	}
+	env := []string{fmt.Sprintf("PGPASSWORD=%s", input.Password)}
+
+	if err := s.executor.Run(timeout, psqlPath, args, backup.CommandOptions{
+		Stdout: &stdout,
+		Stderr: &stderr,
+		Env:    env,
+	}); err != nil {
+		errMsg := strings.TrimSpace(stderr.String())
+		if errMsg == "" {
+			errMsg = err.Error()
+		}
+		return nil, apperror.BadRequest("DATABASE_DISCOVER_PSQL_FAILED", fmt.Sprintf("连接 PostgreSQL 失败：%s", sanitizeMessage(errMsg)), err)
+	}
+
+	skipDBs := map[string]bool{
+		"postgres": true,
+	}
+
+	var databases []string
+	for _, line := range strings.Split(stdout.String(), "\n") {
+		db := strings.TrimSpace(line)
+		if db == "" || skipDBs[db] || strings.HasPrefix(db, "template") {
+			continue
+		}
+		databases = append(databases, db)
+	}
+
+	return &DatabaseDiscoverResult{Databases: databases}, nil
+}