feat(streams): add phase-1 single-server SFU session and transport APIs

Backend/media/sfu/registry.ts

@@ -0,0 +1,41 @@
+import type { SfuSessionDescriptor, SfuSessionState } from './types';
+
+type StoredSfuSession = SfuSessionDescriptor & {
+  updatedAt: string;
+};
+
+export class SfuSessionRegistry {
+  private readonly sessions = new Map<string, StoredSfuSession>();
+
+  get(streamSessionId: string): SfuSessionDescriptor | null {
+    const found = this.sessions.get(streamSessionId);
+    if (!found) return null;
+    const { updatedAt: _updatedAt, ...descriptor } = found;
+    return descriptor;
+  }
+
+  set(session: SfuSessionDescriptor): SfuSessionDescriptor {
+    const now = new Date().toISOString();
+    this.sessions.set(session.streamSessionId, { ...session, updatedAt: now });
+    return session;
+  }
+
+  updateState(streamSessionId: string, state: SfuSessionState): SfuSessionDescriptor | null {
+    const existing = this.sessions.get(streamSessionId);
+    if (!existing) return null;
+
+    const next: StoredSfuSession = {
+      ...existing,
+      state,
+      updatedAt: new Date().toISOString(),
+    };
+    this.sessions.set(streamSessionId, next);
+    const { updatedAt: _updatedAt, ...descriptor } = next;
+    return descriptor;
+  }
+
+  list(): SfuSessionDescriptor[] {
+    return Array.from(this.sessions.values()).map(({ updatedAt: _updatedAt, ...descriptor }) => descriptor);
+  }
+}
+