When a card was added via billing page, it was only stored in the DB — never
attached to the Stripe customer. This meant every charge was on an unattached
PM, which Stripe processes differently.

For subscription upgrades the user is actively present, so off_session=True
was incorrect: it marks the charge as a Merchant-Initiated Transaction (MIT)
which settles slower than Customer-Initiated (CIT). Other sites that use
Stripe's frontend SDK do CITs, which is why their charges clear faster.

- dashboard_add_payment_method_stripe: attach PM to Stripe customer and set
  as customer default immediately when the card is added
- auto_charge: add off_session parameter (default True for existing auto
  top-up/renewal callers); document the distinction
- dashboard_subscribe_tier: pass off_session=False so the upgrade charge is
  processed as a CIT and settles at normal speed