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/application.ex

@@ -1,14 +1,39 @@
 defmodule OrgGarden.Application do
-  @moduledoc false
+  @moduledoc """
+  OTP Application for org-garden.
+
+  Starts core infrastructure services:
+  - Finch HTTP client pool
+  - Process registry for named lookups
+  - DynamicSupervisor for runtime-started services
+  - Health check HTTP server
+
+  Service-mode components (Watcher, Quartz) are started dynamically
+  via OrgGarden.Server when running in serve mode.
+  """
   use Application
 
   @impl true
   def start(_type, _args) do
     children = [
-      {Finch, name: OrgGarden.Finch}
+      # HTTP client pool for Zotero/external requests
+      {Finch, name: OrgGarden.Finch},
+
+      # Process registry for named lookups
+      {Registry, keys: :unique, name: OrgGarden.Registry},
+
+      # Dynamic supervisor for serve mode components
+      {DynamicSupervisor, name: OrgGarden.DynamicSupervisor, strategy: :one_for_one},
+
+      # Health check HTTP server
+      {Bandit, plug: OrgGarden.Health, port: health_port(), scheme: :http}
     ]
 
     opts = [strategy: :one_for_one, name: OrgGarden.AppSupervisor]
     Supervisor.start_link(children, opts)
   end
+
+  defp health_port do
+    Application.get_env(:org_garden, :health_port, 9090)
+  end
 end