Add /health endpoint and CHECKS file for zero-downtime deploys

CHECKS

@@ -0,0 +1,5 @@
+WAIT=5
+TIMEOUT=60
+ATTEMPTS=3
+
+/health ok

lib/microwaveprop_web/controllers/health_controller.ex

@@ -0,0 +1,12 @@
+defmodule MicrowavepropWeb.HealthController do
+  use MicrowavepropWeb, :controller
+
+  def check(conn, _params) do
+    # Verify DB is reachable
+    Ecto.Adapters.SQL.query!(Microwaveprop.Repo, "SELECT 1")
+
+    conn
+    |> put_resp_content_type("text/plain")
+    |> send_resp(200, "ok")
+  end
+end

lib/microwaveprop_web/router.ex

@@ -14,6 +14,9 @@ defmodule MicrowavepropWeb.Router do
     plug :accepts, ["json"]
   end
 
+  # Health check — no pipeline, minimal overhead
+  get "/health", MicrowavepropWeb.HealthController, :check
+
   scope "/", MicrowavepropWeb do
     pipe_through :browser