feat(auth): external sign-in for the website and admin panel

Players sign in to the website, and admins to the admin panel, with an
OpenID Connect provider (DorkNet:SignIn:*). The first sign-in links the
provider identity (issuer + subject) to a DorkNet account with that
account's password; after that the provider sign-in is enough.

- Cookie sessions per host, keys persisted in the database so redeploys
  keep people signed in; unlink, deletion or a ban ends the session.
- Link attempts limited to 5 failures per 15 minutes per identity and
  per username.
- Admin panel: "Sign in with ..." exchanges the session for the usual
  admin tokens via GET /api/admin/v1/sso (IsAdmin still required).
- Downloads: optional DorkNet:Downloads:RequireSignIn serves the
  manifest and files only to signed-in players; the Downloads and Quest
  pages offer sign-in instead.
- Website: sign-in in the header, /link and /account pages.

The game client's login is unchanged.

DorkNet.Server/Controllers/Admin/AdminLoginController.cs

@@ -1,3 +1,4 @@
+using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.EntityFrameworkCore;
 using DorkNet.Server.Data;
@@ -41,6 +42,27 @@ public class AdminLoginController(
         if (!player.IsAdmin)
             return StatusCode(403, new { error = "not_admin" });
 
+        return TokenResponse(player);
+    }
+
+    /// <summary>GET api/admin/v1/sso — exchange a single sign-on session
+    /// (<see cref="ExternalSignIn.WebScheme"/>, signed in on this admin host via
+    /// the external provider) for the same token pair <c>login</c> returns.
+    /// Admin rights still come from the account's <c>IsAdmin</c>.</summary>
+    [HttpGet("sso")]
+    public async Task<ActionResult> Sso()
+    {
+        var web = await HttpContext.AuthenticateAsync(ExternalSignIn.WebScheme);
+        if (!web.Succeeded || ExternalSignIn.PlayerId(web.Principal) is not { } playerId)
+            return Unauthorized(new { error = "not_signed_in" });
+        var player = await db.Players.FirstOrDefaultAsync(p => p.Id == playerId);
+        if (player is null) return Unauthorized(new { error = "not_signed_in" });
+        if (!player.IsAdmin) return StatusCode(403, new { error = "not_admin" });
+        return TokenResponse(player);
+    }
+
+    private OkObjectResult TokenResponse(Data.Entities.PlayerEntity player)
+    {
         var (access, refresh) = authService.GenerateTokenPair(player.Id);
         return Ok(new
         {