feat(media): add phase5 media provider abstraction and stream credentials APIs

Backend/db/schema.ts

@@ -65,6 +65,10 @@ 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(),
+  mediaProvider: varchar('media_provider', { length: 32 }).default('mock').notNull(),
+  mediaSessionId: varchar('media_session_id', { length: 255 }),
+  publishEndpoint: text('publish_endpoint'),
+  subscribeEndpoint: text('subscribe_endpoint'),
   streamKey: varchar('stream_key', { length: 255 }),
   startedAt: timestamp('started_at', { withTimezone: true }),
   endedAt: timestamp('ended_at', { withTimezone: true }),