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/admin-ui/src/components/Sidebar.tsx

@@ -43,6 +43,7 @@ const groups: Array<{ title: string; items: Array<{ to: string; label: string; i
       { to: '/gift',       label: 'Gift items',        icon: <Gift />,   new: true },
       { to: '/passwords',  label: 'Reset passwords',   icon: <KeyRound />, new: true },
       { to: '/grants',     label: 'Grant maker pen',   icon: <Wrench />,   new: true },
+      { to: '/signup-codes', label: 'Signup codes',    icon: <KeyRound />, new: true },
       { to: '/settings',   label: 'Server settings',   icon: <Settings />, new: true },
     ],
   },