add server diagnostics contract

contracts/v1/server-diagnostics-response.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["status", "database", "twitch_identity_count", "unseen_notification_count", "realtime"],
+  "properties": {
+    "status": { "type": "string", "enum": ["healthy"] },
+    "database": { "type": "string", "enum": ["connected"] },
+    "twitch_identity_count": { "type": "integer", "minimum": 0 },
+    "unseen_notification_count": { "type": "integer", "minimum": 0 },
+    "realtime": { "type": "string", "enum": ["available"] }
+  }
+}

fixtures/v1/server-diagnostics.response.json

@@ -0,0 +1,7 @@
+{
+  "status": "healthy",
+  "database": "connected",
+  "twitch_identity_count": 2,
+  "unseen_notification_count": 0,
+  "realtime": "available"
+}