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/postinst

@@ -0,0 +1,47 @@
+#!/bin/bash
+set -e
+
+case "$1" in
+    configure)
+        if ! id syncserver > /dev/null 2>&1; then
+            useradd --system --no-create-home --shell /usr/sbin/nologin syncserver || true
+        fi
+
+        install -d -o syncserver -g syncserver -m 0750 /var/lib/syncserver/data || true
+        install -d -o syncserver -g syncserver -m 0750 /var/lib/syncserver/ssh || true
+        install -d -o syncserver -g syncserver -m 0750 /var/lib/syncserver/logs || true
+        install -d -o root -g root -m 0755 /etc/syncserver || true
+
+        if [ ! -f /etc/syncserver/config.yaml ]; then
+            cat > /etc/syncserver/config.yaml << 'EOF'
+# SyncServer configuration
+# Copy this file and edit as needed.
+# Environment variables SYNCSERVER_* take precedence over this file.
+
+data_dir: /var/lib/syncserver
+addr: :8080
+
+auth:
+  jwt_secret: ""  # Leave empty to generate a random one on startup
+  jwt_expiry_hours: 24
+
+scheduler:
+  timezone: UTC
+EOF
+            chown root:root /etc/syncserver/config.yaml
+            chmod 0644 /etc/syncserver/config.yaml
+        fi
+
+        if command -v systemctl > /dev/null 2>&1; then
+            systemctl daemon-reload 2>/dev/null || true
+            systemctl enable syncserver.service 2>/dev/null || true
+            systemctl start syncserver.service 2>/dev/null || true
+        fi
+        ;;
+
+    abort-upgrade|abort-remove|abort-configure)
+        exit 0
+        ;;
+esac
+
+exit 0