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/api/handlers_ws.go

@@ -0,0 +1,64 @@
+package api
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/syncserver/internal/syncengine"
+)
+
+type SSEHandler struct {
+	engine *syncengine.Engine
+}
+
+func NewSSEHandler(engine *syncengine.Engine) *SSEHandler {
+	return &SSEHandler{engine: engine}
+}
+
+func (h *SSEHandler) Stream(w http.ResponseWriter, r *http.Request) {
+	jobIDStr := r.URL.Query().Get("job_id")
+	var filterJobID int64
+	if jobIDStr != "" {
+		filterJobID, _ = strconv.ParseInt(jobIDStr, 10, 64)
+	}
+
+	flusher, ok := w.(http.Flusher)
+	if !ok {
+		http.Error(w, "SSE not supported", http.StatusInternalServerError)
+		return
+	}
+
+	w.Header().Set("Content-Type", "text/event-stream")
+	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Connection", "keep-alive")
+	w.Header().Set("X-Accel-Buffering", "no")
+
+	flusher.Flush()
+
+	if h.engine == nil {
+		return
+	}
+
+	events := h.engine.Events()
+	for {
+		select {
+		case evt := <-events:
+			if filterJobID != 0 && evt.JobID != filterJobID {
+				continue
+			}
+			data, _ := json.Marshal(evt)
+			fmt.Fprintf(w, "event: %s\ndata: %s\n\n", evt.Type, data)
+			flusher.Flush()
+
+		case <-r.Context().Done():
+			return
+
+		case <-time.After(30 * time.Second):
+			fmt.Fprintf(w, ": keepalive\n\n")
+			flusher.Flush()
+		}
+	}
+}