[17.0][IMP] auto_backup_fs_file: GC-based cleanup, folder visibility, regression test

[rrebollo]

Summary

Port improvements from the v18 migration PR (#3619) back to v17.

Changes

F1+F2 — folder visibility and constrains

• Override folder as required=False so fs_file records do not require a folder path
• Add _check_folder_required_for_method constrains to re-enforce folder requirement only for local/sftp methods
• Hide folder in form view when method == 'fs_file'
• Show responsible_id only when method == 'fs_file'

F3 — explicit GC chain in db.backup.fs.file

• Add @api.depends('db_backup_id.days_to_keep', 'create_date') to _compute_is_expired so recomputation is triggered correctly
• Add explicit unlink() override to ensure fs_attachment GC stack (_fs_mark_for_gc) is triggered when a backup record is deleted
• Remove dead get_fs_storage_filename() method (no longer called)

F4 — cleanup() via GC, not rm_file

• Remove direct fs.rm_file() call in cleanup()
• Physical file deletion now delegated to ir.attachment.unlink() → autovacuum GC

F5 — cleanup() reachable from action_backup()

• Track successful_fs recordset inside action_backup()
• Call successful_fs.cleanup() after super().action_backup()
• Bug: previously cleanup() was never called for fs_file records via the production cron path

F6 — regression test

• New test_action_backup_triggers_cleanup verifying the F5 fix end-to-end
• GC chain assertion added to test_ordinary_flow
• assertFalse(backup_config.folder) added to verify F1

Docs + typos

• Updated ROADMAP, CONTEXT, USAGE with GC-based cleanup explanation
• Fixed :wa typo in INSTALL.md

Related: #3619 (v18), #3621 (v16)
Migration issue: #2752
@BinhexTeam T22571