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/Data/Entities/ExternalLoginEntity.cs

@@ -0,0 +1,29 @@
+namespace DorkNet.Server.Data.Entities;
+
+/// <summary>
+/// An identity from the external sign-in provider (OpenID Connect) linked to a
+/// DorkNet account. One row per (Issuer, Subject): the provider's stable user
+/// id, never the email, which can change or be reused. A player proves they own
+/// the account once, with its password, when the link is made
+/// (<c>ExternalSignInController.Link</c>); after that the provider alone signs
+/// them in to the site and, for admins, the admin panel.
+/// </summary>
+public class ExternalLoginEntity
+{
+    public long Id { get; set; }
+
+    public long PlayerId { get; set; }
+
+    /// <summary>The provider's issuer URL (the <c>iss</c> claim).</summary>
+    public string Issuer { get; set; } = string.Empty;
+
+    /// <summary>The provider's user id (the <c>sub</c> claim).</summary>
+    public string Subject { get; set; } = string.Empty;
+
+    /// <summary>The email the provider reported when the link was made or last
+    /// used. Shown to the player; never used to find the account.</summary>
+    public string Email { get; set; } = string.Empty;
+
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime LastUsedAt { get; set; } = DateTime.UtcNow;
+}