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

packaging/debian/DEBIAN/prerm

@@ -0,0 +1,20 @@
+#!/bin/bash
+set -e
+
+case "$1" in
+    remove|purge|deconfigure)
+        if command -v systemctl > /dev/null 2>&1; then
+            systemctl stop syncserver.service 2>/dev/null || true
+            systemctl disable syncserver.service 2>/dev/null || true
+        fi
+        ;;
+    upgrade)
+        ;;
+    failed-upgrade)
+        ;;
+    *)
+        exit 0
+        ;;
+esac
+
+exit 0