feat(backend): add SIMPLE_STREAMING WebRTC control-path streaming

Backend/db/schema.ts

@@ -65,6 +65,8 @@ export const streamSessions = pgTable('stream_sessions', {
   requesterDeviceId: uuid('requester_device_id').notNull().references(() => devices.id),
   status: varchar('status', { length: 32 }).default('requested').notNull(),
   reason: varchar('reason', { length: 32 }).default('on_demand').notNull(),
+  // Legacy provider-backed fields are retained for compatibility with older sessions.
+  // SIMPLE_STREAMING relies on direct WebRTC signaling and does not populate them.
   mediaProvider: varchar('media_provider', { length: 32 }).default('mock').notNull(),
   mediaSessionId: varchar('media_session_id', { length: 255 }),
   publishEndpoint: text('publish_endpoint'),