init: add contract definitions and database schema with migrations skeleton

contract/ingest/telemetry.schema.json

@@ -0,0 +1,46 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "$id": "contract/ingest/telemetry.schema.json",
+  "allOf": [
+    { "$ref": "contract/ingest/common.schema.json" },
+    {
+      "type": "object",
+      "properties": {
+        "payload": {
+          "type": "object",
+          "required": ["measurements"],
+          "additionalProperties": false,
+          "properties": {
+            "measurements": {
+              "type": "array",
+              "minItems": 1,
+              "items": {
+                "type": "object",
+                "required": ["metric", "value"],
+                "additionalProperties": false,
+                "properties": {
+                  "metric": {
+                    "type": "string",
+                    "minLength": 1
+                  },
+                  "value": {
+                    "type": ["number", "string", "boolean"]
+                  },
+                  "unit": {
+                    "type": "string"
+                  },
+                  "source": {
+                    "type": "string"
+                  },
+                  "ts": {
+                    "type": "integer"
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+  ]
+}