Add service infrastructure for long-running deployment

- Add configuration system (config/*.exs, OrgGarden.Config)
- Refactor supervision tree with DynamicSupervisor and Registry
- Add OrgGarden.Server for serve mode lifecycle management
- Add health check HTTP endpoints (Bandit/Plug on :9090)
- Add telemetry events for export and watcher operations
- Implement graceful shutdown with SIGTERM handling
- Add Mix Release support with overlay scripts
- Add NixOS module for systemd service deployment
- Update documentation with service usage

lib/org_garden/health.ex

@@ -0,0 +1,89 @@
+defmodule OrgGarden.Health do
+  @moduledoc """
+  Health check HTTP endpoints.
+
+  Provides liveness and readiness probes for systemd/kubernetes health checks.
+
+  ## Endpoints
+
+    * `GET /health/live` — Always returns 200 if the process is alive
+    * `GET /health/ready` — Returns 200 if all components are ready, 503 otherwise
+    * `GET /health` — Returns JSON status of all components
+  """
+
+  use Plug.Router
+
+  plug(:match)
+  plug(:dispatch)
+
+  # Liveness probe — is the process alive?
+  get "/health/live" do
+    send_resp(conn, 200, "ok")
+  end
+
+  # Readiness probe — are all components ready to serve?
+  get "/health/ready" do
+    checks = run_checks()
+    all_healthy = Enum.all?(checks, fn {_name, status} -> status == :ok end)
+
+    if all_healthy do
+      send_resp(conn, 200, "ready")
+    else
+      send_resp(conn, 503, "not ready")
+    end
+  end
+
+  # Full health status as JSON
+  get "/health" do
+    checks = run_checks()
+    all_healthy = Enum.all?(checks, fn {_name, status} -> status == :ok end)
+
+    status =
+      if all_healthy do
+        "healthy"
+      else
+        "degraded"
+      end
+
+    body =
+      Jason.encode!(%{
+        status: status,
+        checks:
+          Map.new(checks, fn {name, status} ->
+            {name, Atom.to_string(status)}
+          end)
+      })
+
+    conn
+    |> put_resp_content_type("application/json")
+    |> send_resp(if(all_healthy, do: 200, else: 503), body)
+  end
+
+  match _ do
+    send_resp(conn, 404, "not found")
+  end
+
+  # -------------------------------------------------------------------
+  # Health checks
+  # -------------------------------------------------------------------
+
+  defp run_checks do
+    [
+      {:server, check_server()},
+      {:watcher, check_watcher()},
+      {:quartz, check_quartz()}
+    ]
+  end
+
+  defp check_server do
+    if Process.whereis(OrgGarden.Server), do: :ok, else: :not_running
+  end
+
+  defp check_watcher do
+    if Process.whereis(OrgGarden.Watcher), do: :ok, else: :not_running
+  end
+
+  defp check_quartz do
+    if Process.whereis(OrgGarden.Quartz), do: :ok, else: :not_running
+  end
+end