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/config.ex

@@ -0,0 +1,98 @@
+defmodule OrgGarden.Config do
+  @moduledoc """
+  Centralized configuration access with validation.
+
+  Provides a unified interface for accessing configuration values,
+  with support for defaults and required value validation.
+
+  ## Usage
+
+      OrgGarden.Config.get(:http_port)
+      #=> 8080
+
+      OrgGarden.Config.get!(:quartz_path)
+      #=> "/path/to/quartz" or raises if not set
+
+      OrgGarden.Config.pipeline_opts()
+      #=> %{zotero_url: "...", bibtex_file: nil, citation_mode: :warn}
+  """
+
+  @doc """
+  Get a configuration value with an optional default.
+  """
+  def get(key, default \\ nil) do
+    Application.get_env(:org_garden, key, default)
+  end
+
+  @doc """
+  Get a required configuration value. Raises if not set.
+  """
+  def get!(key) do
+    case Application.get_env(:org_garden, key) do
+      nil -> raise ArgumentError, "Missing required configuration: #{key}"
+      value -> value
+    end
+  end
+
+  @doc """
+  Build pipeline options map for transforms.
+  """
+  def pipeline_opts do
+    %{
+      zotero_url: get(:zotero_url, "http://localhost:23119"),
+      bibtex_file: get(:bibtex_file),
+      citation_mode: get(:citation_mode, :warn)
+    }
+  end
+
+  @doc """
+  Validate that all required configuration is present.
+  Returns :ok or {:error, reasons}.
+  """
+  def validate do
+    errors =
+      []
+      |> validate_quartz_path()
+      |> validate_citation_mode()
+
+    case errors do
+      [] -> :ok
+      errors -> {:error, errors}
+    end
+  end
+
+  @doc """
+  Validate configuration and raise on errors.
+  """
+  def validate! do
+    case validate() do
+      :ok -> :ok
+      {:error, errors} -> raise "Configuration errors: #{inspect(errors)}"
+    end
+  end
+
+  # Private validation helpers
+
+  defp validate_quartz_path(errors) do
+    case get(:quartz_path) do
+      nil ->
+        errors
+
+      path ->
+        cli_path = Path.join(path, "quartz/bootstrap-cli.mjs")
+
+        if File.exists?(cli_path) do
+          errors
+        else
+          [{:quartz_path, "bootstrap-cli.mjs not found at #{cli_path}"} | errors]
+        end
+    end
+  end
+
+  defp validate_citation_mode(errors) do
+    case get(:citation_mode) do
+      mode when mode in [:silent, :warn, :strict] -> errors
+      other -> [{:citation_mode, "Invalid citation mode: #{inspect(other)}"} | errors]
+    end
+  end
+end