feat: 新增 web 端 — axum HTTP 服务 + Docker 化

新增 workspace crate :
- axum 0.8 + tokio 异步 HTTP 服务
- / → 嵌入式 HTML 页面（输入→实时预览→下载/复制）
- /api/qr?text=&level=M&margin=4&size=8 → PNG
- Dockerfile: rust-alpine 多阶段构建，镜像仅 ~12MB
- Cargo.toml: workspace 新增 web 成员

部署:
  docker build -t qrgen-web -f web/Dockerfile .
  docker run -p 3000:3000 qrgen-web

Co-Authored-By: Claude <noreply@anthropic.com>

web/src/main.rs

@@ -0,0 +1,80 @@
+use axum::{
+    extract::Query,
+    http::{header, StatusCode},
+    response::{Html, IntoResponse},
+    routing::get,
+    Router,
+};
+use qr_core::qr::{QrCode, QrConfig, VersionMode};
+use qr_core::version::EcLevel;
+use serde::Deserialize;
+
+/// GET /api/qr 查询参数
+#[derive(Deserialize)]
+struct QrParams {
+    text: String,
+    #[serde(default = "default_level")]
+    level: String,
+    #[serde(default = "default_margin")]
+    margin: u8,
+    #[serde(default = "default_size")]
+    size: u8,
+}
+
+fn default_level() -> String { "M".into() }
+fn default_margin() -> u8 { 4 }
+fn default_size() -> u8 { 8 }
+
+fn parse_level(s: &str) -> Result<EcLevel, String> {
+    match s.to_uppercase().as_str() {
+        "L" => Ok(EcLevel::L),
+        "M" => Ok(EcLevel::M),
+        "Q" => Ok(EcLevel::Q),
+        "H" => Ok(EcLevel::H),
+        _ => Err(format!("无效纠错级别: {}，支持 L/M/Q/H", s)),
+    }
+}
+
+/// 首页 HTML（编译期嵌入）
+async fn index() -> Html<&'static str> {
+    Html(include_str!("templates/index.html"))
+}
+
+/// QR 码生成 API → PNG 图片
+async fn generate_qr(Query(params): Query<QrParams>) -> impl IntoResponse {
+    let level = match parse_level(&params.level) {
+        Ok(l) => l,
+        Err(e) => return (StatusCode::BAD_REQUEST, e).into_response(),
+    };
+
+    let config = QrConfig {
+        level,
+        version: VersionMode::Auto,
+        margin: params.margin,
+    };
+
+    let qr = match QrCode::encode(&params.text, config) {
+        Ok(q) => q,
+        Err(e) => return (StatusCode::BAD_REQUEST, e).into_response(),
+    };
+
+    let png = match qr.to_png_bytes(params.size) {
+        Ok(b) => b,
+        Err(e) => return (StatusCode::INTERNAL_SERVER_ERROR, e.to_string()).into_response(),
+    };
+
+    ([(header::CONTENT_TYPE, "image/png")], png).into_response()
+}
+
+#[tokio::main]
+async fn main() {
+    let app = Router::new()
+        .route("/", get(index))
+        .route("/api/qr", get(generate_qr));
+
+    let addr = "0.0.0.0:3000";
+    println!("QRGen Web → http://{}", addr);
+
+    let listener = tokio::net::TcpListener::bind(addr).await.unwrap();
+    axum::serve(listener, app).await.unwrap();
+}