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

config/config.exs

@@ -0,0 +1,18 @@
+import Config
+
+# Default configuration values
+# These are overridden by config/runtime.exs in releases
+
+config :org_garden,
+  zotero_url: "http://localhost:23119",
+  citation_mode: :warn,
+  http_port: 8080,
+  ws_port: 3001,
+  health_port: 9090
+
+config :logger, :console,
+  format: "$time $metadata[$level] $message\n",
+  metadata: [:module]
+
+# Import environment specific config
+import_config "#{config_env()}.exs"