signup codes: admin-issued invite codes + site /join redemption

Port the December signup-code feature to the 2020.03 branch.

While account creation is disabled, the only way in is now an
admin-issued single-use signup code, redeemed on the public site's /join
page. Because the 2020.03 watch login is device-id-bound with no code
field, a new device refused at the signups-disabled gate (PlatformLogin /
account create) is stashed in a PendingDevices table keyed by the real
client IP (CF-Connecting-IP aware). The /join page shows the player the
device their own client just reported; they enter code + username and the
account is minted bound to that device, so the next launch logs straight
in. No watch changes.

- SignupCodeEntity + PendingDeviceEntity; tables created via idempotent
  CREATE TABLE IF NOT EXISTS at boot (Program.cs Postgres patch block +
  SQLite path) since they post-date the migration chain.
- SignupCodeService: generate / list / revoke / redeem + pending-device
  capture (single-use, expiry, revoke validation).
- Admin: GET/POST signup-codes + revoke endpoints and a Signup codes
  admin-ui page (generate with descriptor/expiry, status table, revoke).
- Public: anonymous /api/site/v1/join pending-devices + redeem, and a
  /join site page (+ header link).
- README documents the flow.

DorkNet.Server/Data/Entities/SignupCodeEntity.cs

@@ -0,0 +1,40 @@
+namespace DorkNet.Server.Data.Entities;
+
+/// <summary>
+/// An admin-issued, single-use signup code (Discord-invite style). While
+/// the server has signups disabled, the only way to mint a new account is
+/// to redeem one of these on the public site's <c>/join</c> page. Each
+/// code carries a free-text <see cref="Descriptor"/> so admins can track
+/// who they handed it to, an optional <see cref="ExpiresAt"/>, and the
+/// redemption result (<see cref="RedeemedByPlayerId"/> /
+/// <see cref="RedeemedAt"/>). Codes can be revoked before use.
+/// </summary>
+public class SignupCodeEntity
+{
+    public long Id { get; set; }
+
+    /// <summary>The opaque code string the player types on /join.
+    /// Uppercase, hyphenated, unambiguous alphabet (no 0/O/1/I).</summary>
+    public string Code { get; set; } = string.Empty;
+
+    /// <summary>Free-text note for the admin — "who is this for". Shown
+    /// only in the admin panel, never to the redeemer.</summary>
+    public string Descriptor { get; set; } = string.Empty;
+
+    /// <summary>Player id of the admin who generated the code (0 if
+    /// generated by a system/CLI path).</summary>
+    public long CreatedByPlayerId { get; set; }
+
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    /// <summary>Optional hard expiry. Null = never expires.</summary>
+    public DateTime? ExpiresAt { get; set; }
+
+    /// <summary>Set once the code is redeemed; null while unused.</summary>
+    public long? RedeemedByPlayerId { get; set; }
+    public DateTime? RedeemedAt { get; set; }
+
+    /// <summary>Admin kill switch — a revoked code can never be redeemed,
+    /// even if otherwise valid.</summary>
+    public bool Revoked { get; set; }
+}