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/App.tsx

@@ -21,6 +21,7 @@ import { Gift } from './pages/Gift';
 import { Passwords } from './pages/Passwords';
 import { Grants } from './pages/Grants';
 import { Settings } from './pages/Settings';
+import { SignupCodes } from './pages/SignupCodes';
 
 export default function App() {
   return (
@@ -54,6 +55,7 @@ export default function App() {
           <Route path="passwords"  element={<Passwords />} />
           <Route path="grants"     element={<Grants />} />
           <Route path="settings"     element={<Settings />} />
+          <Route path="signup-codes" element={<SignupCodes />} />
         </Route>
         <Route path="*" element={<Navigate to="/" replace />} />
       </Routes>