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/seed.go

@@ -0,0 +1,37 @@
+package auth
+
+import (
+	"database/sql"
+	"log/slog"
+)
+
+func SeedAdmin(db *sql.DB, username, password string) error {
+	if username == "" || password == "" {
+		return nil
+	}
+
+	var exists bool
+	err := db.QueryRow("SELECT EXISTS(SELECT 1 FROM users WHERE username = ?)", username).Scan(&exists)
+	if err != nil {
+		return err
+	}
+	if exists {
+		return nil
+	}
+
+	hash, err := HashPassword(password)
+	if err != nil {
+		return err
+	}
+
+	_, err = db.Exec(
+		"INSERT INTO users (username, password_hash, role) VALUES (?, ?, ?)",
+		username, string(hash), "admin",
+	)
+	if err != nil {
+		return err
+	}
+
+	slog.Info("admin user created", "username", username)
+	return nil
+}