[ZhuchkaTriplesix]

Closes #1497. Part of the Beta channel epic #1499.

A beta can carry a newer local database schema than the stable it is promoted to, and nothing protected the user from that, or told a package-manager user that the channel setting does nothing for them.

Backup before an in-app update

• UpdateBackup (lib/core/storage/update_backup.dart) copies querya.db and its -wal to querya.db.pre-<from>-<to>.bak, keeps the newest 3 (a copy's -wal goes with it) and deletes older ones. Versions are sanitised in the name.
• LocalDb.backupBeforeUpdate closes the database, makes the copy and leaves reopening to the next query.
• The update dialog calls it right before installing (after the unsaved-work confirmation). If it fails, a dialog says so and offers Cancel or Install without a copy.

Newer-schema guard

• LocalDb passes onDowngrade and throws LocalDbNewerSchemaException(found, supported, path). The throw happens inside sqflite's open transaction, so it rolls back and the file keeps its version number; without it sqflite stamps the older number on the file and the build carries on with tables it does not know. The recovery screen already prints the error; the message names the newer data format and the .bak to restore.

Release channel row (Preferences → General)

• On a Snap, Flatpak, AUR, .deb or .rpm install (UpdateInstallContext.isPackageManagerInstall) the dropdown is disabled, shows Stable and the hint says updates come from the package manager (stable only).
• AppUpdaterService.checkForUpdates reads releases/latest there whatever the stored channel is, so such an install is not told about a beta it cannot install.

Docs: docs/user-guide.md ("Updates, betas and going back": channels, the pre-update copy, how to go back by restoring it) and docs/security.md (schema version and the guard).

Not done: a stored "schema version" separate from sqflite's user_version (it already is the schema version: _dbVersion = 13); a button in the recovery screen that restores the newest .bak (the file name is in the message and the user guide describes the manual steps).

Tests: backup naming, WAL copy and removal of a stale WAL copy, keeping the newest three with their WALs, a missing database; LocalDb backup then reopen; a database set to a newer user_version is refused with the right numbers and its version is still the newer one afterwards; the channel row on a portable and on a package install; a package install reading releases/latest while the stored channel is Beta; the update dialog copies before installing, asks when the copy fails, and honours Cancel and Install without a copy.