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/auth/jwt.go

@@ -0,0 +1,69 @@
+package auth
+
+import (
+	"errors"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+var (
+	ErrInvalidToken = errors.New("invalid token")
+	ErrExpiredToken = errors.New("token expired")
+)
+
+type Claims struct {
+	UserID   int64  `json:"user_id"`
+	Username string `json:"username"`
+	Role     string `json:"role"`
+	jwt.RegisteredClaims
+}
+
+type JWTManager struct {
+	secret     []byte
+	expiryH    int
+}
+
+func NewJWTManager(secret string, expiryH int) *JWTManager {
+	return &JWTManager{
+		secret:  []byte(secret),
+		expiryH: expiryH,
+	}
+}
+
+func (m *JWTManager) Generate(userID int64, username, role string) (string, time.Time, error) {
+	expiresAt := time.Now().Add(time.Duration(m.expiryH) * time.Hour)
+	claims := &Claims{
+		UserID:   userID,
+		Username: username,
+		Role:     role,
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(expiresAt),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+			NotBefore: jwt.NewNumericDate(time.Now()),
+		},
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	signed, err := token.SignedString(m.secret)
+	return signed, expiresAt, err
+}
+
+func (m *JWTManager) Validate(tokenStr string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenStr, &Claims{}, func(token *jwt.Token) (interface{}, error) {
+		if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, ErrInvalidToken
+		}
+		return m.secret, nil
+	})
+	if err != nil {
+		if errors.Is(err, jwt.ErrTokenExpired) {
+			return nil, ErrExpiredToken
+		}
+		return nil, ErrInvalidToken
+	}
+	claims, ok := token.Claims.(*Claims)
+	if !ok || !token.Valid {
+		return nil, ErrInvalidToken
+	}
+	return claims, nil
+}