Multi-account support and streaming fixes

- Change LinkedAccount unique constraint to (userId, serviceId, accountId)
- Add linkedAccountId to StreamDestination for per-account targeting
- OAuth callbacks upsert by accountId so different accounts create new rows
- Delete endpoint changed to /providers/accounts/:id
- getDecryptedToken resolves tokens by linkedAccountId instead of serviceId
- /start transition wrapped in try-catch (enableAutoStart compatibility)
- /end always attempts YouTube complete transition regardless of plan status
- autoDetectEndedPlans loads tokens per-destination

prisma/schema.prisma

@@ -49,7 +49,7 @@ model LinkedAccount {
   createdAt       DateTime @default(now())
   updatedAt       DateTime @updatedAt
 
-  @@unique([userId, serviceId])
+  @@unique([userId, serviceId, accountId])
   @@index([userId])
 }
 
@@ -67,19 +67,20 @@ model StreamPlan {
 }
 
 model StreamDestination {
-  id            String     @id @default(uuid())
-  planId        String
-  plan          StreamPlan @relation(fields: [planId], references: [id], onDelete: Cascade)
-  serviceId     String
-  title         String
-  description   String     @default("")
-  privacyStatus String     @default("public")
-  gameId        String     @default("")
-  tags          String     @default("")
-  rtmpUrl       String     @default("")
-  streamKey     String     @default("")
-  broadcastId   String     @default("")
-  status        String     @default("PENDING")
+  id              String     @id @default(uuid())
+  planId          String
+  plan            StreamPlan @relation(fields: [planId], references: [id], onDelete: Cascade)
+  serviceId       String
+  linkedAccountId String     @default("")
+  title           String
+  description     String     @default("")
+  privacyStatus   String     @default("public")
+  gameId          String     @default("")
+  tags            String     @default("")
+  rtmpUrl         String     @default("")
+  streamKey       String     @default("")
+  broadcastId     String     @default("")
+  status          String     @default("PENDING")
 
   @@index([planId])
 }