feat(passwordless): pass3: passwordless account (via emailed magic-link)

Backend
  - RegisterDto — username and password both become Option<String> with #[serde(default)] so JSON can omit them entirely.
  - AuthApplicationService::register — username uniqueness check skipped when None (multiple NULLs OK under the UNIQUE index); password hashing skipped when None; User::new called with the actual Options instead of forcing Some(...).
  - auth_handler::register — branches on dto.password.is_none(). With password → existing 201 + UserDto. Without → triggers MagicLinkInviteService::send_login_link(&email) best-effort, then returns 200 + {"message": "Check your email…"}. The
  OIDC-mode-disables-password-registration gate now only fires for the password path (email-only signup is still allowed even in OIDC-only mode, because it doesn't store a password).
  - magic_link_handler::redirect_target — new 3-way decision tree:
    - Resource target (folder invitation) → /#/files/folder/{id} (existing)
    - NULL resource + is_external = false → /#/files (the welcome path for new internal users — they have a home folder)
    - NULL resource + is_external = true → /#/sharedwithme (the existing external-user landing)

  Tests
  - New tests/api/registration.hurl with 9 requests covering: classic (with-password) register → 201 + UserDto, email-only register → 200 + uniform message + welcome magic-link captured, redemption → 302 to /#/files + cookies set, profile read → username
  absent + is_external: false, resend magic-link works (eligible while passwordless), cleanup deletes both new users.
  - Wired into tests/api/run.sh right after auth_login.hurl.

  Plan additions
  - auth-simplification.md gained PR 22 at the bottom of the PR sequence — device-bound magic-link redemption via challenge cookie + asymmetric TTLs (login: 10 min, invitation: 24 h). Full design recap, schema migration, config knobs
  (OXICLOUD_MAGIC_LINK_LOGIN_TTL_MINUTES / _INVITE_TTL_HOURS), and Hurl coverage outline are in the plan. Slots in before PR 21's docs so the architecture page describes the final state from the start.

  Checks — cargo fmt, cargo clippy --all-features --all-targets -- -D warnings, cargo test --lib (297 passed), biome, stylelint, tsc, full Hurl suite (16 files) all green.

src/application/dtos/user_dto.rs

@@ -80,9 +80,21 @@ pub struct LoginDto {
 
 #[derive(Debug, Serialize, Deserialize, Clone, ToSchema)]
 pub struct RegisterDto {
-    pub username: String,
+    /// Optional handle (2-64 chars, no `@`). When omitted, the user can
+    /// claim one later via the profile-edit endpoint. Users without a
+    /// username cannot use NextCloud clients or create app passwords
+    /// (Basic-Auth resolves users by username); web UI / native API
+    /// works fine without one.
+    #[serde(default)]
+    pub username: Option<String>,
     pub email: String,
-    pub password: String,
+    /// Optional password (≥8 chars when present). When omitted, a
+    /// welcome magic-link is mailed to `email` for first-session
+    /// bootstrap. The user can later set a password via the
+    /// change-password endpoint to switch to classic username/email +
+    /// password login.
+    #[serde(default)]
+    pub password: Option<String>,
 }
 
 /// DTO for the one-time initial admin setup endpoint (`/api/setup`).