R3 two-factor sign-in: TOTP with recovery codes, asked for only after a correct password

Completion program R3 (docs/ROADMAP.md, 2026-10-17).

- Services/TwoFactor.cs: TOTP (RFC 6238, 30 s, 6 digits, SHA-1, one step of slack, Otp.NET 1.4.1) and ten recovery
  codes (SHA-256 kept, pulled when used). RootUser.Totp holds the secret, the last step taken (moved only by a
  conditional update, so a code works once even for two sign-ins at the same moment) and the codes' hashes.
- /clientapi/user/totp/setup (the password) answers the secret, its otpauth:// URI and a QR code as an SVG data URI
  (QRCoder 1.8.0); /totp/enable takes the app's first code, shows the recovery codes once and ends every other session
  (this device gets a new one); /totp/disable takes the password and a code. GET /clientapi/user/account says whether
  it is on and how many recovery codes are left.
- Sign-in (/clientapi/user/login, the invitation login, /oauth/login) asks for LoginForm.OneTimeCode with a 428, and
  only once the password was right; the OAuth page has the field. Recovery by email leaves two-factor on.
- PrivaPub admin reset-totp <root>: the way back for a login that lost its authenticator and its codes.
- scenarios/accounts.sh checks it all, with codes computed in Python; the OAuth page with its antiforgery token.

Verified: dotnet build PrivaPub.sln -c Release; the pasture (gts, mailpit): accounts 45/45.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WpVCoDeM1qUTRp4fXQdfof

PrivaPub/PrivaPub.csproj

@@ -19,6 +19,8 @@
     <PackageReference Include="NetVips.Native.linux-x64" Version="8.18.7" />
     <PackageReference Include="OpenIddict.AspNetCore" Version="7.7.1" />
     <PackageReference Include="OpenIddict.MongoDb" Version="7.7.1" />
+    <PackageReference Include="Otp.NET" Version="1.4.1" />
+    <PackageReference Include="QRCoder" Version="1.8.0" />
     <PackageReference Include="Serilog.AspNetCore" Version="10.0.0" />
     <PackageReference Include="Serilog.Sinks.MongoDB" Version="7.3.0" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="10.2.3" />