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/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 />} />