feat(auth): support RS256 + OIDC discovery (JWKS auto-fetch)

Add Jwt__Algorithm config to choose between HS256 (shared symmetric key,
existing behavior, default) and RS256 (Authority-based OIDC discovery,
public-key auto-fetch with periodic refresh).

RS256 mode makes the server compatible with any standard OAuth 2.1 / OIDC
provider (Logto, ZITADEL, Keycloak, Auth0) without requiring a shared
secret. HS256 mode remains the default for minimal self-built AS setups.

Auth/JwtBearerSetup.cs

@@ -6,8 +6,12 @@ using System.Text;
 namespace GiteaMcp.Auth;
 
 /// <summary>
-/// JWT Bearer 验签配置，HS256 对称密钥方案。
-/// ValidIssuer / ValidAudience 由配置驱动；支持 Current + Previous 双密钥（轮换窗口）。
+/// JWT Bearer 验签配置。根据 JwtOptions.Algorithm 选择：
+/// <list type="bullet">
+///   <item><c>HS256</c>：与 AS 共享对称密钥（Current + Previous 双密钥用于轮换）。</item>
+///   <item><c>RS256</c>：委托 ASP.NET Core 通过 OIDC discovery 拉 JWKS，
+///     自动刷新公钥，AS 端密钥轮换无需重启本服务。</item>
+/// </list>
 /// </summary>
 public static class JwtBearerSetup
 {
@@ -18,6 +22,8 @@ public static class JwtBearerSetup
         var jwtOpts = configuration.GetSection(JwtOptions.SectionName).Get<JwtOptions>()
             ?? new JwtOptions();
 
+        var algorithm = (jwtOpts.Algorithm ?? "HS256").Trim().ToUpperInvariant();
+
         services
             .AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
             .AddJwtBearer(options =>
@@ -26,7 +32,7 @@ public static class JwtBearerSetup
                 // ClaimTypes.NameIdentifier 之类的长 URI，下游 FindFirst("scope") 取不到。
                 options.MapInboundClaims = false;
 
-                options.TokenValidationParameters = new TokenValidationParameters
+                var tvp = new TokenValidationParameters
                 {
                     ValidateIssuer = true,
                     ValidIssuer = jwtOpts.Issuer,
@@ -35,28 +41,51 @@ public static class JwtBearerSetup
                     ValidAudience = jwtOpts.Audience,
 
                     ValidateIssuerSigningKey = true,
-                    // ToList 物化一次，避免每次验签重建 SymmetricSecurityKey。
-                    IssuerSigningKeys = BuildSigningKeys(jwtOpts).ToList(),
-
                     ValidateLifetime = true,
                     // 允许 2 分钟时钟偏差，缓解 AS 与本服务时钟漂移
                     ClockSkew = TimeSpan.FromMinutes(2),
                 };
+
+                if (algorithm == "RS256")
+                {
+                    if (string.IsNullOrWhiteSpace(jwtOpts.Issuer))
+                        throw new InvalidOperationException(
+                            "Jwt:Issuer 未配置，RS256 模式无法启动。");
+
+                    // Authority = Issuer 时，JwtBearer 会自动从
+                    // <Issuer>/.well-known/openid-configuration 拉 OIDC 元数据，
+                    // 再从其中的 jwks_uri 拉公钥并周期性刷新。IssuerSigningKeys
+                    // 由 ConfigurationManager 在验签时动态注入，无需在此设置。
+                    options.Authority = jwtOpts.Issuer;
+                    options.RequireHttpsMetadata = !IsLocalhostUrl(jwtOpts.Issuer);
+                }
+                else if (algorithm == "HS256")
+                {
+                    // ToList 物化一次，避免每次验签重建 SymmetricSecurityKey。
+                    tvp.IssuerSigningKeys = BuildHs256Keys(jwtOpts).ToList();
+                }
+                else
+                {
+                    throw new InvalidOperationException(
+                        $"Jwt:Algorithm '{jwtOpts.Algorithm}' 不支持。可选值：HS256, RS256");
+                }
+
+                options.TokenValidationParameters = tvp;
             });
 
         return services;
     }
 
     /// <summary>
-    /// 构建 Current + Previous 双密钥列表，供 SDK 依次尝试验签。
+    /// 构建 Current + Previous 双 HS256 密钥列表，供 SDK 依次尝试验签。
     /// 轮换期间两把钥匙同时有效，过期的旧 Token 会被 ValidateLifetime 自然拦截。
     /// Current 未配置直接抛错，避免容器静默以"任何 token 都不通过"的状态运行。
     /// </summary>
-    private static IEnumerable<SecurityKey> BuildSigningKeys(JwtOptions opts)
+    private static IEnumerable<SecurityKey> BuildHs256Keys(JwtOptions opts)
     {
         if (string.IsNullOrWhiteSpace(opts.SigningKey.Current))
             throw new InvalidOperationException(
-                "Jwt:SigningKey:Current 未配置，gitea-mcp 无法启动。" +
+                "Jwt:SigningKey:Current 未配置，HS256 模式无法启动。" +
                 "请通过 env Jwt__SigningKey__Current 注入与 auth server 共享的 HS256 密钥。");
 
         yield return new SymmetricSecurityKey(Encoding.UTF8.GetBytes(opts.SigningKey.Current));
@@ -64,4 +93,8 @@ public static class JwtBearerSetup
         if (!string.IsNullOrWhiteSpace(opts.SigningKey.Previous))
             yield return new SymmetricSecurityKey(Encoding.UTF8.GetBytes(opts.SigningKey.Previous));
     }
+
+    private static bool IsLocalhostUrl(string url) =>
+        url.StartsWith("http://localhost", StringComparison.OrdinalIgnoreCase) ||
+        url.StartsWith("http://127.0.0.1", StringComparison.OrdinalIgnoreCase);
 }