feat(media): add single-server SFU scaffolding and media mode config

Backend/media/config.ts

@@ -0,0 +1,43 @@
+export type MediaMode = 'legacy' | 'single_server_sfu';
+
+const parseMediaMode = (value: string | undefined): MediaMode => {
+  const normalized = (value ?? 'legacy').trim().toLowerCase();
+  if (normalized === 'single_server_sfu') {
+    return 'single_server_sfu';
+  }
+  return 'legacy';
+};
+
+const parseCsv = (value: string | undefined): string[] => {
+  if (!value) return [];
+  return value
+    .split(',')
+    .map((item) => item.trim())
+    .filter(Boolean);
+};
+
+const parsePositiveNumber = (value: string | undefined): number | null => {
+  if (!value) return null;
+  const parsed = Number(value);
+  if (!Number.isFinite(parsed) || parsed <= 0) {
+    return null;
+  }
+  return parsed;
+};
+
+export const mediaMode: MediaMode = parseMediaMode(process.env.MEDIA_MODE);
+
+export const mediaConfig = {
+  mode: mediaMode,
+  turn: {
+    urls: parseCsv(process.env.TURN_URLS),
+    username: process.env.TURN_USERNAME ?? '',
+    credential: process.env.TURN_CREDENTIAL ?? '',
+  },
+  recordingsDir: process.env.MEDIA_RECORDINGS_DIR ?? 'media-recordings',
+  limits: {
+    maxPublishers: parsePositiveNumber(process.env.MEDIA_MAX_PUBLISHERS),
+    maxSubscribersPerRoom: parsePositiveNumber(process.env.MEDIA_MAX_SUBSCRIBERS_PER_ROOM),
+  },
+};
+