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

nix/module.nix

@@ -0,0 +1,148 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.org-garden;
+in
+{
+  options.services.org-garden = {
+    enable = lib.mkEnableOption "org-garden publishing service";
+
+    package = lib.mkOption {
+      type = lib.types.package;
+      description = "The org-garden package to use.";
+    };
+
+    notesDir = lib.mkOption {
+      type = lib.types.path;
+      description = "Path to org-roam notes directory.";
+    };
+
+    outputDir = lib.mkOption {
+      type = lib.types.path;
+      default = "/var/lib/org-garden";
+      description = "Output directory for generated content.";
+    };
+
+    port = lib.mkOption {
+      type = lib.types.port;
+      default = 8080;
+      description = "HTTP server port.";
+    };
+
+    wsPort = lib.mkOption {
+      type = lib.types.port;
+      default = 3001;
+      description = "WebSocket hot reload port.";
+    };
+
+    healthPort = lib.mkOption {
+      type = lib.types.port;
+      default = 9090;
+      description = "Health check endpoint port.";
+    };
+
+    zoteroUrl = lib.mkOption {
+      type = lib.types.str;
+      default = "http://localhost:23119";
+      description = "Zotero Better BibTeX URL.";
+    };
+
+    bibtexFilePath = lib.mkOption {
+      type = lib.types.nullOr lib.types.path;
+      default = null;
+      description = "Path to fallback BibTeX file.";
+    };
+
+    citationMode = lib.mkOption {
+      type = lib.types.enum [ "silent" "warn" "strict" ];
+      default = "warn";
+      description = "Citation resolution failure mode.";
+    };
+
+    user = lib.mkOption {
+      type = lib.types.str;
+      default = "org-garden";
+      description = "User to run the service as.";
+    };
+
+    group = lib.mkOption {
+      type = lib.types.str;
+      default = "org-garden";
+      description = "Group to run the service as.";
+    };
+
+    openFirewall = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = "Whether to open the firewall for the HTTP port.";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    users.users.${cfg.user} = {
+      isSystemUser = true;
+      group = cfg.group;
+      home = cfg.outputDir;
+      createHome = true;
+    };
+
+    users.groups.${cfg.group} = { };
+
+    systemd.services.org-garden = {
+      description = "Org-Garden Publishing Service";
+      documentation = [ "https://github.com/ignacio.ballesteros/org-garden" ];
+      wantedBy = [ "multi-user.target" ];
+      after = [ "network.target" ];
+
+      environment = {
+        NOTES_DIR = toString cfg.notesDir;
+        OUTPUT_DIR = cfg.outputDir;
+        PORT = toString cfg.port;
+        WS_PORT = toString cfg.wsPort;
+        HEALTH_PORT = toString cfg.healthPort;
+        ZOTERO_URL = cfg.zoteroUrl;
+        CITATION_MODE = cfg.citationMode;
+      } // lib.optionalAttrs (cfg.bibtexFile != null) {
+        BIBTEX_FILE = toString cfg.bibtexFile;
+      };
+
+      serviceConfig = {
+        Type = "exec";
+        ExecStart = "${cfg.package}/bin/org-garden serve";
+        Restart = "on-failure";
+        RestartSec = 5;
+
+        # Directories
+        StateDirectory = "org-garden";
+        WorkingDirectory = cfg.outputDir;
+
+        # User/Group
+        User = cfg.user;
+        Group = cfg.group;
+
+        # Hardening
+        NoNewPrivileges = true;
+        ProtectSystem = "strict";
+        ProtectHome = "read-only";
+        ReadWritePaths = [ cfg.outputDir ];
+        ReadOnlyPaths = [ cfg.notesDir ];
+        PrivateTmp = true;
+        PrivateDevices = true;
+        ProtectKernelTunables = true;
+        ProtectKernelModules = true;
+        ProtectControlGroups = true;
+        RestrictAddressFamilies = [ "AF_INET" "AF_INET6" "AF_UNIX" ];
+        RestrictNamespaces = true;
+        LockPersonality = true;
+        MemoryDenyWriteExecute = false;  # Required for BEAM JIT
+        RestrictRealtime = true;
+        RestrictSUIDSGID = true;
+        RemoveIPC = true;
+      };
+    };
+
+    networking.firewall = lib.mkIf cfg.openFirewall {
+      allowedTCPPorts = [ cfg.port ];
+    };
+  };
+}