feat: add dry-run mode, GFS retention policies, and notifications

- Add --dry-run/-n flag for backup commands with comprehensive preflight checks
  - Database connectivity validation
  - Required tools availability check
  - Storage target and permissions verification
  - Backup size estimation
  - Encryption and cloud storage configuration validation

- Implement GFS (Grandfather-Father-Son) retention policies
  - Daily/Weekly/Monthly/Yearly tier classification
  - Configurable retention counts per tier
  - Custom weekly day and monthly day settings
  - ISO week handling for proper week boundaries

- Add notification system with SMTP and webhook support
  - SMTP email notifications with TLS/STARTTLS
  - Webhook HTTP notifications with HMAC-SHA256 signing
  - Slack-compatible webhook payload format
  - Event types: backup/restore started/completed/failed, cleanup, verify, PITR
  - Configurable severity levels and retry logic

- Update README.md with documentation for all new features

internal/checks/preflight_test.go

@@ -0,0 +1,134 @@
+package checks
+
+import (
+	"testing"
+)
+
+func TestPreflightResult(t *testing.T) {
+	result := &PreflightResult{
+		Checks:    []PreflightCheck{},
+		AllPassed: true,
+		DatabaseInfo: &DatabaseInfo{
+			Type:    "postgres",
+			Version: "PostgreSQL 15.0",
+			Host:    "localhost",
+			Port:    5432,
+			User:    "postgres",
+		},
+		StorageInfo: &StorageInfo{
+			Type:           "local",
+			Path:           "/backups",
+			AvailableBytes: 10 * 1024 * 1024 * 1024,
+			TotalBytes:     100 * 1024 * 1024 * 1024,
+		},
+		EstimatedSize: 1 * 1024 * 1024 * 1024,
+	}
+
+	if !result.AllPassed {
+		t.Error("Result should be AllPassed")
+	}
+
+	if result.DatabaseInfo.Type != "postgres" {
+		t.Errorf("DatabaseInfo.Type = %q, expected postgres", result.DatabaseInfo.Type)
+	}
+
+	if result.StorageInfo.Path != "/backups" {
+		t.Errorf("StorageInfo.Path = %q, expected /backups", result.StorageInfo.Path)
+	}
+}
+
+func TestPreflightCheck(t *testing.T) {
+	check := PreflightCheck{
+		Name:    "Database Connectivity",
+		Status:  StatusPassed,
+		Message: "Connected successfully",
+		Details: "PostgreSQL 15.0",
+	}
+
+	if check.Status != StatusPassed {
+		t.Error("Check status should be passed")
+	}
+
+	if check.Name != "Database Connectivity" {
+		t.Errorf("Check name = %q", check.Name)
+	}
+}
+
+func TestCheckStatusString(t *testing.T) {
+	tests := []struct {
+		status   CheckStatus
+		expected string
+	}{
+		{StatusPassed, "PASSED"},
+		{StatusFailed, "FAILED"},
+		{StatusWarning, "WARNING"},
+		{StatusSkipped, "SKIPPED"},
+	}
+
+	for _, tc := range tests {
+		result := tc.status.String()
+		if result != tc.expected {
+			t.Errorf("Status.String() = %q, expected %q", result, tc.expected)
+		}
+	}
+}
+
+func TestFormatPreflightReport(t *testing.T) {
+	result := &PreflightResult{
+		Checks: []PreflightCheck{
+			{Name: "Test Check", Status: StatusPassed, Message: "OK"},
+		},
+		AllPassed: true,
+		DatabaseInfo: &DatabaseInfo{
+			Type:    "postgres",
+			Version: "PostgreSQL 15.0",
+			Host:    "localhost",
+			Port:    5432,
+		},
+		StorageInfo: &StorageInfo{
+			Type:           "local",
+			Path:           "/backups",
+			AvailableBytes: 10 * 1024 * 1024 * 1024,
+		},
+	}
+
+	report := FormatPreflightReport(result, "testdb", false)
+	if report == "" {
+		t.Error("Report should not be empty")
+	}
+}
+
+func TestFormatPreflightReportPlain(t *testing.T) {
+	result := &PreflightResult{
+		Checks: []PreflightCheck{
+			{Name: "Test Check", Status: StatusFailed, Message: "Connection failed"},
+		},
+		AllPassed:    false,
+		FailureCount: 1,
+	}
+
+	report := FormatPreflightReportPlain(result, "testdb")
+	if report == "" {
+		t.Error("Report should not be empty")
+	}
+}
+
+func TestFormatPreflightReportJSON(t *testing.T) {
+	result := &PreflightResult{
+		Checks:    []PreflightCheck{},
+		AllPassed: true,
+	}
+
+	report, err := FormatPreflightReportJSON(result, "testdb")
+	if err != nil {
+		t.Errorf("FormatPreflightReportJSON() error = %v", err)
+	}
+
+	if len(report) == 0 {
+		t.Error("Report should not be empty")
+	}
+
+	if report[0] != '{' {
+		t.Error("Report should start with '{'")
+	}
+}