init: add retention config step to setup wizard
Add a "your recordings" retention configuration step between observers
(step 4) and get started (step 6) in the first-time setup wizard.
Users choose between keep-forever, keep-N-days, and delete-after-
processing modes with day presets (7/30/90/custom).
finalize() includes retention_mode and retention_days in the POST
payload; init_finalize() writes them as retention.raw_media and
retention.raw_media_days to journal.json. Default is days/7 when
untouched.
Remove the hard-coded retention paragraph from the welcome screen
since users now configure retention during setup.