init: add contract definitions and database schema with migrations skeleton

protocol/schema.json

@@ -1,12 +1,31 @@
 {
   "type": "object",
-  "required": ["v", "id", "type", "ts", "deviceId", "payload"],
+  "required": ["v", "type", "ts", "deviceId", "payload"],
   "properties": {
-    "v": { "type": "integer" },
-    "id": { "type": "string" },
-    "type": { "type": "string" },
-    "ts": { "type": "integer" },
-    "deviceId": { "type": "string" },
-    "payload": { "type": "object" }
+    "v": {
+      "type": "integer",
+      "description": "Protocol version"
+    },
+
+    "type": {
+      "type": "string",
+      "description": "Message type"
+    },
+
+    "ts": {
+      "type": "integer",
+      "description": "Unix time in milliseconds"
+    },
+
+    "deviceId": {
+      "type": "string",
+      "minLength": 1,
+      "description": "Unique device identifier"
+    },
+
+    "payload": {
+      "type": "object",
+      "description": "Message-specific data"
+    }
   }
 }