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

While account creation is disabled, the only way in is now an
admin-issued single-use signup code (Discord-invite style), redeemed on
the public site's /join page.

Because the 2020 watch login is device-id-bound with no code field, the
flow captures it server-side instead: 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 then shows the player the device their own client just
reported, they enter their code + username, and the account is minted
bound to that device — so the next launch logs straight in via the
existing device-id path. No watch changes.

- SignupCodeEntity (code, descriptor, optional expiry, redeemer) +
  PendingDeviceEntity; idempotent CREATE TABLE in DatabaseBootstrap so
  both providers pick them up without a new migration.
- 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 + the post-Initial table-create approach.

DorkNet.Server/Data/DorkNetDbContext.cs

@@ -72,6 +72,8 @@ public class DorkNetDbContext(DbContextOptions<DorkNetDbContext> options) : DbCo
     public DbSet<ClubCategoryTagEntity> ClubCategoryTags => Set<ClubCategoryTagEntity>();
     public DbSet<ClubCategoryAssignmentEntity> ClubCategoryAssignments => Set<ClubCategoryAssignmentEntity>();
     public DbSet<ClubSubscriptionEntity> ClubSubscriptions => Set<ClubSubscriptionEntity>();
+    public DbSet<SignupCodeEntity> SignupCodes => Set<SignupCodeEntity>();
+    public DbSet<PendingDeviceEntity> PendingDevices => Set<PendingDeviceEntity>();
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {