infra(spikes): add Flask callback receiver for LIT-2888 EC2 spike

5-line Flask app on port 3333 that logs callbacks from EC2 user-data and
SSM RunCommand. Used by the EC2 provisioning spike to time when boot
and hydrate callbacks arrive.

infra/spikes/callback_receiver.py

@@ -0,0 +1,48 @@
+"""
+Flask callback receiver for the EC2 provisioning spike (LIT-2888).
+
+Listens on port 3333 for POST /spike. Logs every callback with timestamp + body.
+Used in tandem with ngrok to give EC2 instances a public HTTPS URL to hit.
+
+Run:
+    uv run python infra/spikes/callback_receiver.py
+"""
+
+import json
+import sys
+import time
+from flask import Flask, request
+
+app = Flask(__name__)
+
+# In-memory log of received callbacks. Useful when the spike script polls
+# the receiver to confirm both `phase=boot` and `phase=hydrate` arrived.
+CALLBACKS: list = []
+
+
+@app.post("/spike")
+def spike() -> tuple[dict, int]:
+    body = request.get_json(silent=True) or {}
+    entry = {"received_at": time.time(), "body": body}
+    CALLBACKS.append(entry)
+    print(
+        f"[{time.strftime('%H:%M:%S')}] callback: {json.dumps(body)}",
+        file=sys.stderr,
+        flush=True,
+    )
+    return {"ok": True}, 200
+
+
+@app.get("/callbacks")
+def list_callbacks() -> dict:
+    """Return all callbacks received so far. The spike script polls this."""
+    return {"callbacks": CALLBACKS}
+
+
+@app.get("/health")
+def health() -> dict:
+    return {"ok": True}
+
+
+if __name__ == "__main__":
+    app.run(host="0.0.0.0", port=3333, debug=False)