Implement V2 key rotation support (#7766)

* Implement V2 key rotation support

* Clear user key ids left stale by key rotations

* Refuse key ids that aren't 32 lowercase hex characters, like upstream

* Save the signature key pair before the v2 account state

* Gate v2 logins on upstream's v2 encryption check

* Document the v2 encryption feature flags

* Remove the v2 upgrade feature flags

With force-upgrade-v2-encryption on, the clients loop on any v1 account that stays v1 on a vault route: the migration check runs fullSync(false), which writes lastSync, which triggers the scheduler again. No server response stops it, so don't serve the flag until the clients fix it.

pm-30144-sdk-key-rotation and pm-31050-no-logout-key-upgrade-rotation only serve that upgrade, so they go too.

* Move the v2 upgrade token migration to its own version

Diesel identifies a migration by the date in its folder name alone, and silently drops one of two folders that share it. The Custom Roles migration uses the same date.

* Refuse rotations from a replaced user key, like upstream

Both rotation endpoints accept the optional oldUserKeyId. When it and the
stored key id are both set and differ, the rotation is refused with a 409
asking the client to sync, before the account keys are checked or anything
is written.

Also correct the comment on saving the signature key pair first: by then the
vault data is already re-encrypted, so a failure there doesn't keep the
account usable.

* Update the v2 client minimum in .env.template and the key pair comments

V2 accounts need 2026.10.1, or 2026.10.2 for the desktop app, so the per-flag versions are only enough to create one. The comments on saving the signature key pair now cover the rotation of an account that is already v2.

.env.template

@@ -411,6 +411,9 @@
 ## - "biometrics-sdk-ipc": Enable biometric unlock over the SDK IPC framework (Desktop >= 2026.9.0, Browser >= 2026.9.0)
 ## - "windows-native-credential-sync": Register the desktop app as a passkey provider in Windows, so passkeys in the vault can be used by other apps and browsers.
 ##   Requires the Microsoft Store (appx) build of the desktop app. Stable releases up to and including 2026.9.1 lack the required plugin registration, so it has no effect there yet.
+## - "pm-27278-v2-password-registration": Register new accounts with v2 encryption. (Web >= 2026.6.4, Desktop/Browser/CLI >= 2026.7.0, Android >= 2026.4.1, iOS >= 2026.5.1)
+## - "enable-account-encryption-v2-jit-password-registration": Give SSO users who set their master password on their first login v2 encryption. (Web >= 2026.1.1, Desktop/Browser/CLI >= 2026.1.0, Android >= 2026.4.1, iOS >= 2026.6.0)
+##   These two flags can't be undone: a v2 account stays v2, and it can only log in from clients that are 2026.10.1 or newer (2026.10.2 for the desktop app), so the versions above are only enough to create one.
 # EXPERIMENTAL_CLIENT_FEATURE_FLAGS=
 
 ## Require new device emails. When a user logs in an email is required to be sent.