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

README.org

@@ -7,7 +7,7 @@ An [[https://orgmode.org/][org-roam]] to static website publishing pipeline. Con
 #+begin_example
 org-garden serve  <notes-dir>   # dev server with live reload
 org-garden build  <notes-dir>   # production static build
-org-garden export <notes-dir>   # org → markdown only
+org-garden export <notes-dir>   # org -> markdown only
 #+end_example
 
 * Running with Nix (recommended)
@@ -26,3 +26,52 @@ mix escript.build
 #+end_src
 
 Requires =QUARTZ_PATH= to point to a Quartz install with =node_modules= for =serve= and =build= commands.
+
+* NixOS Service
+
+A NixOS module is provided for running org-garden as a systemd service:
+
+#+begin_src nix
+{
+  inputs.org-garden.url = "github:ignacio.ballesteros/org-garden";
+
+  outputs = { self, nixpkgs, org-garden }: {
+    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
+      modules = [
+        org-garden.nixosModules.default
+        {
+          services.org-garden = {
+            enable = true;
+            package = org-garden.packages.x86_64-linux.default;
+            notesDir = /path/to/notes;
+            port = 8080;
+          };
+        }
+      ];
+    };
+  };
+}
+#+end_src
+
+* Health Checks
+
+When running in serve mode, health endpoints are available on port 9090:
+
+- =GET /health/live= — liveness probe (always 200)
+- =GET /health/ready= — readiness probe (200 if all components ready)
+- =GET /health= — JSON status of all components
+
+* Environment Variables
+
+| Variable       | Default                   | Description                      |
+|----------------+---------------------------+----------------------------------|
+| =QUARTZ_PATH=  | (required for serve/build)| Path to Quartz installation      |
+| =NODE_PATH=    | =node=                    | Node.js executable               |
+| =NOTES_DIR=    | (cli arg)                 | Source notes directory           |
+| =OUTPUT_DIR=   | =.=                       | Output base directory            |
+| =ZOTERO_URL=   | =http://localhost:23119=  | Zotero Better BibTeX URL         |
+| =BIBTEX_FILE=  | (none)                    | Fallback BibTeX file             |
+| =CITATION_MODE=| =warn=                    | =silent=, =warn=, or =strict=    |
+| =PORT=         | =8080=                    | HTTP server port                 |
+| =WS_PORT=      | =3001=                    | WebSocket hot reload port        |
+| =HEALTH_PORT=  | =9090=                    | Health check endpoint port       |