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/site/src/App.tsx

@@ -6,6 +6,7 @@ import { PhotoDetail } from './pages/PhotoDetail';
 import { Players } from './pages/Players';
 import { PlayerProfile } from './pages/PlayerProfile';
 import { Rooms } from './pages/Rooms';
+import { Join } from './pages/Join';
 import { NotFound } from './pages/NotFound';
 
 export function App() {
@@ -20,6 +21,7 @@ export function App() {
           <Route path="/players" element={<Players />} />
           <Route path="/players/:id" element={<PlayerProfile />} />
           <Route path="/rooms" element={<Rooms />} />
+          <Route path="/join" element={<Join />} />
           {/* Legacy paths from the vanilla-JS feed.localhost site */}
           <Route path="/p/:id" element={<RedirectPhoto />} />
           <Route path="/u/:id" element={<RedirectPlayer />} />