chore: add Nix flake and NixOS module support (#20597)

* feat(nix): add flake module and offline-friendly build

* chore(nix): upgrade to Node.js 24 and pnpm 10

Update devenv.nix to match project dependencies:
- Upgrade Node.js from 22 to 24 (aligns with Dockerfile node:24-bookworm)
- Upgrade pnpm from 9 to 10 (aligns with package.json pnpm@10.22.0)

* test: fix buffer-get test timeout

Replace external URL with mock server endpoint to prevent test timeout.
The test was trying to fetch from http://example.com which is not mocked,
causing it to timeout. Now uses http://rsshub.test/headers which is
properly mocked in the test setup.

Author: DzmingLi

.envrc

@@ -0,0 +1 @@
+use flake .

.gitignore

@@ -1,6 +1,6 @@
 .DS_Store
 .cursorrules
-.env*
+.env
 .eslintcache
 .idea
 .log

devenv.nix

@@ -0,0 +1,108 @@
+{ pkgs, lib, config, ... }:
+
+{
+  # https://devenv.sh/basics/
+  env = {
+    NODE_ENV = "dev";
+    NODE_OPTIONS = "--max-http-header-size=32768";
+  };
+
+  # https://devenv.sh/packages/
+  packages = with pkgs; [
+    git
+
+    # Optional: Uncomment if you need browser automation
+    # chromium
+  ];
+
+  # https://devenv.sh/languages/
+  languages.javascript = {
+    enable = true;
+    package = pkgs.nodejs_24;
+    pnpm = {
+      enable = true;
+      package = pkgs.pnpm_10;
+    };
+  };
+
+  # https://devenv.sh/services/
+  services.redis = {
+    enable = lib.mkDefault false;  # Disabled by default, users can enable in devenv.local.nix
+    port = 6379;
+  };
+
+  # https://devenv.sh/scripts/
+  scripts.rsshub-dev.exec = ''
+    pnpm run dev
+  '';
+
+  scripts.rsshub-build.exec = ''
+    pnpm run build
+  '';
+
+  scripts.rsshub-start.exec = ''
+    pnpm start
+  '';
+
+  scripts.rsshub-test.exec = ''
+    pnpm test
+  '';
+
+  # https://devenv.sh/processes/
+  processes = {
+    # Uncomment to auto-start RSSHub in dev mode when entering the shell
+    # rsshub.exec = "pnpm run dev";
+
+    # Example: Auto-start with Redis
+    # rsshub.exec = "pnpm run dev";
+  };
+
+  # https://devenv.sh/pre-commit-hooks/
+  pre-commit.hooks = {
+    # Lint staged files
+    eslint = {
+      enable = true;
+      entry = lib.mkForce "pnpm run format:staged";
+    };
+  };
+
+  enterShell = ''
+    echo ""
+    echo "🚀 RSSHub Development Environment"
+    echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+    echo ""
+    echo "Node.js:  $(node --version)"
+    echo "pnpm:     $(pnpm --version)"
+    ${lib.optionalString config.services.redis.enable ''
+    echo "Redis:    Running on port ${toString config.services.redis.port}"
+    ''}
+    echo ""
+    echo "Available commands:"
+    echo "  rsshub-dev     - Start development server (pnpm run dev)"
+    echo "  rsshub-build   - Build the project (pnpm run build)"
+    echo "  rsshub-start   - Start production server (pnpm start)"
+    echo "  rsshub-test    - Run tests (pnpm test)"
+    ${lib.optionalString (!config.services.redis.enable) ''
+    echo ""
+    echo "💡 Tip: Enable Redis by creating devenv.local.nix:"
+    echo "   { services.redis.enable = true; }"
+    ''}
+    echo ""
+    echo "Documentation: https://docs.rsshub.app"
+    echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+    echo ""
+
+    # Install dependencies if node_modules doesn't exist
+    if [ ! -d "node_modules" ]; then
+      echo "📦 Installing dependencies..."
+      pnpm install
+    fi
+  '';
+
+  # https://devenv.sh/integrations/dotenv/
+  dotenv.enable = true;  # Automatically load .env file
+
+  # Load local overrides if they exist
+  # Users can create devenv.local.nix for personal customizations
+  imports = lib.optional (builtins.pathExists ./devenv.local.nix) ./devenv.local.nix;
+}