fix(auth): count a passkey as the second factor on the managed-profile acts

An account whose only strong credential is a primary passkey was refused
auth.stepup.mfa_not_enrolled on every action behind requireFreshMfa, so it
could not create, edit, hand over or delete a managed profile or manage its
guardians, although a passkey sign-in or re-proof already stamps the
session the gate reads.

requireFreshMfa now defaults to the any-possession proof source: a passkey
makes the gate reachable, and a passkey re-proof through the dialog clears
it. The refusal offers passkey in meta.methods, never the password.

hasSecondFactorEnrolled keeps its narrow meaning on purpose. It decides
whether the conditional gates apply at all (account deletion, the data
reset, the password change, the export, offering manage access, the
recent-proof branches and the native Bearer rules), so none of those
change, and password sign-in still challenges only TOTP and security keys.
The MFA-management routes keep the second-factor source. The decision and
its limits are written beside FreshFactorProofSource.

Copy, the OpenAPI description and the sharing and key-rotation docs say
"second factor or passkey" where this gate applies.

Refs #959

src/lib/api-handler.ts

@@ -1253,11 +1253,13 @@ export type FreshMfaContext = AuthContext & { mfaVerifiedAt: Date };
  * the route is protecting.
  *
  *   `second-factor` — a confirmed TOTP secret or a registered second-factor
- *   security key. The default, and the right question for the routes that MANAGE
- *   a second factor: there is nothing there to protect if nothing is enrolled.
+ *   security key. The default of `requireMfaManagementAuth`, and the right
+ *   question for the routes that MANAGE a second factor: there is nothing there
+ *   to protect if nothing is enrolled.
  *
- *   `any-possession` — the above, OR a registered PRIMARY passkey. The right
- *   question for a route protecting the passkey itself, because a passkey is a
+ *   `any-possession` — the above, OR a registered PRIMARY passkey. The default
+ *   of `requireFreshMfa` since v1.43.3, and the right question for a route
+ *   protecting the passkey itself, because a passkey is a
  *   possession factor in its own right and this codebase already treats it as
  *   one in three places: `/api/auth/passkey/login-verify` stamps `mfaVerifiedAt`
  *   on a passkey login (v1.23, M-review M1), `passkey` is in
@@ -1270,6 +1272,37 @@ export type FreshMfaContext = AuthContext & { mfaVerifiedAt: Date };
  */
 export type FreshFactorProofSource = "second-factor" | "any-possession";
 
+/*
+ * A PASSKEY COUNTS AS A SECOND FACTOR FOR THE UNCONDITIONAL STEP-UP GATE.
+ *
+ * Maintainer decision, v1.43.3. `requireFreshMfa` defaults to
+ * `any-possession`, so an account whose only strong credential is a passkey
+ * is no longer refused `mfa_not_enrolled` on the routes that call it directly
+ * (creating, editing, deleting and handing over a managed profile, inviting
+ * and removing a guardian, the admin key rotation). It clears the gate the
+ * way every other account does: a passkey sign-in or a passkey re-proof at
+ * `POST /api/auth/reproof` stamps `Session.mfaVerifiedAt`, and the gate reads
+ * that stamp inside the same five minutes.
+ *
+ * The limits, each deliberate:
+ *
+ *   - `hasSecondFactorEnrolled` keeps its narrow meaning. It decides whether
+ *     the CONDITIONAL gates (`requireFreshMfaIfEnrolled`,
+ *     `requireFreshMfaOrElevationIfEnrolled`, `requireRecentProof`'s branches,
+ *     the enrollment proofs) apply at all. Widening it would newly gate
+ *     account deletion, the data reset, the password change and the export
+ *     for every passkey holder, take the password re-proof away from them, and
+ *     change what the native app's Bearer requests need. None of that is part
+ *     of the decision.
+ *   - Password login is untouched. It asks for a second factor from
+ *     `totpConfirmedAt` and the security-key count only; a passkey-only
+ *     account signing in with its password is not challenged, as before.
+ *   - The MFA-management routes (`requireMfaManagementAuth`) still default to
+ *     `second-factor`: disabling TOTP or rotating recovery codes on an account
+ *     with no second factor has nothing to protect.
+ *   - Cookie only, as ever. The Bearer path is not reached from here.
+ */
+
 /**
  * v1.23 — step-up gate. Passes only for a COOKIE session whose
  * `Session.mfaVerifiedAt` is within `maxAgeSeconds` AND whose user holds a
@@ -1285,12 +1318,13 @@ export type FreshFactorProofSource = "second-factor" | "any-possession";
  * waves gate account deletion, key rotation, and passphrase export on it.
  *
  * @param proofSource which credentials count as "can prove a factor". Defaults
- *   to the second-factor set every existing caller means; see
- *   {@link FreshFactorProofSource}.
+ *   to `any-possession`, so a primary passkey counts (see the decision note
+ *   below {@link FreshFactorProofSource}); `requireMfaManagementAuth` passes
+ *   `second-factor` explicitly.
  */
 export async function requireFreshMfa(
   maxAgeSeconds: number,
-  proofSource: FreshFactorProofSource = "second-factor",
+  proofSource: FreshFactorProofSource = "any-possession",
 ): Promise<FreshMfaContext> {
   const sessionData = await getSession();
   if (!sessionData)