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/sshmanager/knownhosts.go

@@ -0,0 +1,143 @@
+package sshmanager
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+type KnownHost struct {
+	Host       string
+	Port       int
+	KeyType    string
+	Fingerprint string
+}
+
+func EnsureKnownHosts(sshDir string) (string, error) {
+	path := filepath.Join(sshDir, "known_hosts")
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDONLY, 0644)
+	if err != nil {
+		return "", err
+	}
+	f.Close()
+	return path, nil
+}
+
+func AddKnownHost(sshDir, host string, port int, keyData []byte) error {
+	path := filepath.Join(sshDir, "known_hosts")
+	f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	addr := host
+	if port != 22 {
+		addr = fmt.Sprintf("[%s]:%d", host, port)
+	}
+
+	line := fmt.Sprintf("%s %s\n", addr, strings.TrimSpace(string(keyData)))
+	if _, err := f.WriteString(line); err != nil {
+		return err
+	}
+	return nil
+}
+
+func parseHostPort(entry string) (string, int) {
+	if strings.HasPrefix(entry, "[") {
+		var h string
+		var p int
+		if n, _ := fmt.Sscanf(entry, "[%[^]]]:%d", &h, &p); n == 2 {
+			return h, p
+		}
+	}
+	parts := strings.Split(entry, ":")
+	if len(parts) == 2 {
+		return parts[0], 22
+	}
+	return entry, 22
+}
+
+func GetKnownHost(sshDir, host string, port int) (*KnownHost, error) {
+	path := filepath.Join(sshDir, "known_hosts")
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var targetAddr string
+	if port != 22 {
+		targetAddr = fmt.Sprintf("[%s]:%d", host, port)
+	} else {
+		targetAddr = host
+	}
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Text()
+		parts := strings.Fields(line)
+		if len(parts) < 2 {
+			continue
+		}
+		h, p := parseHostPort(parts[0])
+		if (h == host || parts[0] == targetAddr) && p == port {
+			return &KnownHost{
+				Host:       h,
+				Port:       p,
+				KeyType:    parts[1],
+				Fingerprint: parts[1] + " " + parts[2],
+			}, nil
+		}
+	}
+	return nil, nil
+}
+
+func HasKnownHost(sshDir, host string, port int) (bool, error) {
+	kh, err := GetKnownHost(sshDir, host, port)
+	if err != nil {
+		return false, err
+	}
+	return kh != nil, nil
+}
+
+func RemoveKnownHost(sshDir, host string, port int) error {
+	path := filepath.Join(sshDir, "known_hosts")
+	f, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	var lines []string
+	targetAddr := host
+	if port != 22 {
+		targetAddr = fmt.Sprintf("[%s]:%d", host, port)
+	}
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Text()
+		h, p := parseHostPort(line)
+		if h == host && p == port {
+			continue
+		}
+		if line == targetAddr {
+			continue
+		}
+		lines = append(lines, line)
+	}
+
+	tmp := path + ".tmp"
+	wf, err := os.Create(tmp)
+	if err != nil {
+		return err
+	}
+	for _, l := range lines {
+		wf.WriteString(l + "\n")
+	}
+	wf.Close()
+	return os.Rename(tmp, path)
+}