feat: initial open-source release of Pipely

OTA update server designed for high-concurrency, low-bandwidth deployments.
GORM-backed PostgreSQL, JWT auth, device management, artefact versioning,
deployment rollout with gated rate-limited downloads, and a React admin panel.

internal/download/limiter.go

@@ -0,0 +1,19 @@
+package download
+
+// GlobalDownloadGate wraps a GatedSemaphore for download-specific concurrency control.
+// See internal/middleware/rate_limit.go for the underlying GatedSemaphore implementation.
+//
+// The gate ensures that the total number of concurrent download streams never exceeds
+// the configured limit (default 50), which keeps total bandwidth usage under 10 Mbps
+// when each stream is throttled to ~25 KB/s.
+type GlobalDownloadGate struct {
+	sem chan struct{}
+}
+
+func NewGlobalDownloadGate(maxConcurrent int) *GlobalDownloadGate {
+	return &GlobalDownloadGate{sem: make(chan struct{}, maxConcurrent)}
+}
+
+func (g *GlobalDownloadGate) Acquire()       { g.sem <- struct{}{} }
+func (g *GlobalDownloadGate) Release()       { <-g.sem }
+func (g *GlobalDownloadGate) Available() int { return cap(g.sem) - len(g.sem) }