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

@@ -44,6 +44,7 @@ const groups: Array<{ title: string; items: Array<{ to: string; label: string; i
       { to: '/passwords',  label: 'Reset passwords',   icon: <KeyRound />, new: true },
       { to: '/grants',     label: 'Grant maker pen',   icon: <Wrench />,   new: true },
       { to: '/storage',    label: 'Storage backfill',  icon: <HardDrive />, new: true },
+      { to: '/signup-codes', label: 'Signup codes',    icon: <KeyRound />, new: true },
       { to: '/settings',   label: 'Server settings',   icon: <Settings />, new: true },
     ],
   },