feat(ws): opt-in allowed origins and observer key (#170)

config.yaml.example

@@ -149,6 +149,9 @@ websocket:
   # Within this rate budget, a full concurrent cap accepts then closes with 1013
   # before hello, allowing browsers to back off without evicting existing clients.
   max_connects_per_minute: 10
+  # Other sites allowed to open /ws. Exact scheme://host[:port], no wildcards.
+  #allowed_origins:
+  #  - https://example.com
 
 # Node staleness, deletion, and clock-drift thresholds.
 #nodes: