add subscribing and unsubscribing to threads

migrations.lua

@@ -102,4 +102,15 @@ return {
     
     db.query("CREATE INDEX idx_rate_limit_user_method ON api_rate_limits (user_id, method)")
   end,
+  
+  [13] = function ()
+    schema.create_table("subscriptions", {
+      {"id", types.integer{primary_key = true}},
+      {"user_id", "INTEGER REFERENCES users(id) ON DELETE CASCADE"},
+      {"thread_id", "INTEGER REFERENCES threads(id) ON DELETE CASCADE"},
+      {"last_seen", "INTEGER DEFAULT (unixepoch(CURRENT_TIMESTAMP)) NOT NULL"},
+    })
+    
+    db.query("CREATE INDEX idx_subscription_user_thread ON subscriptions (user_id, thread_id)")
+  end,
 }