feat: complete SyncServer implementation

Full-stack Go monolith with embedded React frontend for orchestrating
rsync-over-SSH file synchronization with Wake-on-LAN support.

Features:
- JWT auth (HS256) with bcrypt password hashing
- CRUD for machines (with WoL config) and sync_pairs
- Ed25519 SSH key generation and known_hosts management
- WoL magic packet sender + TCP-connect waiter with backoff
- Sync engine: rsync subprocess, per-pair job queue, progress parsing
- Homebrew cron parser for scheduled syncs
- SSE stream for live job status (queued/waking_up/running/success/failed)
- React+TS+Vite+Tailwind SPA embedded via embed.FS
- Debian packaging with systemd unit, postinst/prerm/postrm

Tech stack:
- Go 1.22+ (CGO_ENABLED=0, pure SQLite via modernc.org/sqlite)
- chi router for HTTP API
- TypeScript + React 18 + Tailwind CSS frontend
- Cross-compiled to Linux amd64 for Proxmox LXC deployment

Tests: wol (MAC parsing, magic packet), syncengine/queue, scheduler/cron

internal/scheduler/cron_test.go

@@ -0,0 +1,97 @@
+package scheduler
+
+import (
+	"testing"
+	"time"
+)
+
+func TestParseCron(t *testing.T) {
+	tests := []struct {
+		expr    string
+		wantErr bool
+	}{
+		{"* * * * *", false},
+		{"0 * * * *", false},
+		{"*/5 * * * *", false},
+		{"0,30 * * * *", false},
+		{"0-30 * * * *", false},
+		{"*/15 9-17 * * *", false},
+		{"0 0 1 * *", false},
+		{"0 0 * * 0", false},
+		{"0 0 1,15 * *", false},
+		{"invalid", true},
+		{"* * * *", true},
+		{"60 * * * *", true},
+		{"* 24 * * *", true},
+	}
+
+	for _, tt := range tests {
+		_, err := ParseCron(tt.expr)
+		if (err != nil) != tt.wantErr {
+			t.Errorf("ParseCron(%q) error = %v, wantErr %v", tt.expr, err, tt.wantErr)
+		}
+	}
+}
+
+func TestCronMatches(t *testing.T) {
+	expr, err := ParseCron("*/5 * * * *")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tests := []struct {
+		minute int
+		want   bool
+	}{
+		{0, true},
+		{5, true},
+		{10, true},
+		{15, true},
+		{1, false},
+		{2, false},
+		{7, false},
+	}
+
+	now := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+	for _, tt := range tests {
+		tm := time.Date(now.Year(), now.Month(), now.Day(), now.Hour(), tt.minute, 0, 0, time.UTC)
+		if got := expr.Matches(tm); got != tt.want {
+			t.Errorf("Matches(minute=%d) = %v, want %v", tt.minute, got, tt.want)
+		}
+	}
+}
+
+func TestCronMatchesSpecific(t *testing.T) {
+	expr, err := ParseCron("30 9 15 * *")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	matches := time.Date(2024, 6, 15, 9, 30, 0, 0, time.UTC)
+	if !expr.Matches(matches) {
+		t.Error("should match 9:30 on 15th of month")
+	}
+
+	notMatch := time.Date(2024, 6, 16, 9, 30, 0, 0, time.UTC)
+	if expr.Matches(notMatch) {
+		t.Error("should not match 9:30 on 16th of month")
+	}
+}
+
+func TestNextRun(t *testing.T) {
+	expr, err := ParseCron("*/5 * * * *")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	from := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+	next := NextRun(expr, from)
+
+	if next.Minute() != 5 || next.Hour() != 12 {
+		t.Errorf("NextRun = %v, want 12:05", next)
+	}
+
+	if !next.After(from) {
+		t.Error("NextRun should be after from time")
+	}
+}