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/process/mediamtx.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"os"
 	"os/exec"
 	"sync"
 	"time"
@@ -18,6 +19,7 @@ type MediaMTXManager struct {
 	cmd        *exec.Cmd
 	binaryPath string
 	configPath string
+	extraEnv   []string
 	startTime  time.Time
 	lastError  string
 	stopCh     chan struct{}
@@ -32,6 +34,14 @@ func NewMediaMTXManager(binaryPath, configPath string) *MediaMTXManager {
 	}
 }
 
+// SetEnv sets additional environment variables (e.g. MTX_HLSVARIANT) that are
+// passed to MediaMTX. Must be called before Start.
+func (m *MediaMTXManager) SetEnv(env []string) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.extraEnv = env
+}
+
 // Start launches the MediaMTX process. It auto-restarts on unexpected exit.
 func (m *MediaMTXManager) Start() error {
 	m.mu.Lock()
@@ -52,6 +62,9 @@ func (m *MediaMTXManager) startLocked() error {
 	}
 
 	m.cmd = exec.Command(m.binaryPath, m.configPath)
+	if len(m.extraEnv) > 0 {
+		m.cmd.Env = append(os.Environ(), m.extraEnv...)
+	}
 	m.lastError = ""
 	m.stopped = false
 	m.stopCh = make(chan struct{})