[16.0] [BACKPORT] [MIG] auto_backup_fs_file

[rrebollo]

Summary

Backport of auto_backup_fs_file from v17 to v16, with all improvements from the v18 migration (PR #3619) included.

Commits

Original migration commits

• [ADD] auto_backup_fs_file: initial module
• [MIG] auto_backup_fs_file: Migration to 16.0 (backport)

Improvement commits (backport from v18/PR #3619)

F1+F2 — folder visibility and constrains

• Override folder as required=False for fs_file records
• Add _check_folder_required_for_method constrains (re-enforces requirement for local/sftp only)
• Hide folder in form view when method == 'fs_file' (v16 attrs= syntax)
• 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
• Add explicit unlink() override to trigger fs_attachment GC stack
• Remove dead get_fs_storage_filename() method

F4 — cleanup() via GC, not rm_file

• Remove direct fs.rm_file() call; delegate to ir.attachment.unlink()

F5 — cleanup() reachable from action_backup()

• Track successful_fs and call cleanup() from action_backup()

F6 — regression test

• test_action_backup_triggers_cleanup regression test
• GC chain assertion in test_ordinary_flow
• assertFalse(backup_config.folder) assertion

Docs + typos

• Updated ROADMAP, CONTEXT, USAGE; fixed INSTALL.md :wa typo

Related: #3619 (v18), #3683 (v17)

Migrations issue #2412
@BinhexTeam T22571