Add Device/Video models, signaling WebSocket, device and content routes

- Prisma: Device model (Quest/Phone, online status, battery, storage,
  game/streaming/cortex state), Video model with likes, VideoLike
- Signaling WebSocket for SDP/ICE relay and device presence
- Device routes: list, status, delete
- Content routes: video CRUD with range-support streaming
- SignalingManager service for device socket registry and heartbeat

prisma/schema.prisma

@@ -25,6 +25,9 @@ model User {
   followers      Follow[]        @relation("following")
   following      Follow[]        @relation("follower")
   likes          Like[]
+  devices        Device[]
+  videos         Video[]
+  videoLikes     VideoLike[]
 }
 
 model PairingCode {
@@ -130,3 +133,54 @@ model Like {
   @@unique([userId, planId])
   @@index([planId])
 }
+
+model Device {
+  id               String   @id @default(uuid())
+  userId           String
+  user             User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  deviceId         String
+  deviceName       String   @default("")
+  deviceType       String   @default("QUEST") // "QUEST" or "PHONE"
+  isOnline         Boolean  @default(false)
+  lastSeen         DateTime @default(now())
+  batteryLevel     Int?
+  storageAvailable Int?
+  runningGame      String?
+  streamingState   String?
+  cortexState      String?
+  createdAt        DateTime @default(now())
+  updatedAt        DateTime @updatedAt
+
+  @@unique([userId, deviceId])
+  @@index([userId])
+}
+
+model Video {
+  id           String      @id @default(uuid())
+  userId       String
+  user         User        @relation(fields: [userId], references: [id], onDelete: Cascade)
+  title        String
+  description  String      @default("")
+  duration     Int         @default(0)
+  thumbnailUrl String?
+  videoUrl     String
+  fileSize     Int?
+  isPublic     Boolean     @default(true)
+  createdAt    DateTime    @default(now())
+  updatedAt    DateTime    @updatedAt
+  likes        VideoLike[]
+
+  @@index([userId])
+}
+
+model VideoLike {
+  id        String   @id @default(uuid())
+  userId    String
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  videoId   String
+  video     Video    @relation(fields: [videoId], references: [id], onDelete: Cascade)
+  createdAt DateTime @default(now())
+
+  @@unique([userId, videoId])
+  @@index([videoId])
+}