Tuner v1: HLS pipeline with playback/ingest modes

FFmpeg -> MediaMTX -> HLS relay with runtime-configurable modes:
- PLAYBACK_MODE: standard (mpegts), fmp4 (HEVC), llhls
- INGEST_MODE: rtmp, rtsp
- INGEST_AUDIO_MODE: aac, copy
- Hourly playlist auto-refresh from upstream URL
- Fullscreen video UI with collapsible sidebar, upstream-down banner
- Same-origin HLS proxy for HTTPS deployments

Tagged as the last HLS-based version before the mpegts.js (v2) rework.

backend/api/playlist.go

@@ -0,0 +1,35 @@
+package api
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+)
+
+// FetchPlaylist downloads a playlist from url and writes it to destPath.
+func FetchPlaylist(url, destPath string) error {
+	log.Printf("[playlist] fetching from %s", url)
+	resp, err := http.Get(url) //nolint:gosec
+	if err != nil {
+		return fmt.Errorf("fetch playlist: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("fetch playlist: server returned %s", resp.Status)
+	}
+	if err := os.MkdirAll("/data", 0o755); err != nil {
+		return fmt.Errorf("mkdir /data: %w", err)
+	}
+	f, err := os.Create(destPath)
+	if err != nil {
+		return fmt.Errorf("create playlist file: %w", err)
+	}
+	defer f.Close()
+	if _, err := io.Copy(f, resp.Body); err != nil {
+		return fmt.Errorf("write playlist file: %w", err)
+	}
+	log.Printf("[playlist] saved to %s", destPath)
+	return nil
+}