Extract the AI-categorization upgrade dialog into a shared UpgradeDialog
(plan picker -> Stripe checkout) and use it at two more Pro-feature entry
points that previously showed a plain "go to billing" dialog:
- Connections: "Connect Bank"
- Accounts: the "Connected" account option
Each passes its own copy and an upsell "source". To measure how much revenue
each point drives, the source is captured two ways:
- PostHog "upgrade_checkout_started" event at click (intent), and
- carried into Stripe as subscription metadata (SubscriptionController), then
persisted onto the local "subscriptions.upsell_source" column when the
subscription webhook lands (PersistUpsellSourceFromStripe) so revenue can be
grouped by point with the existing local tooling.
Delete the now-unused UpgradeConnectionDialog.
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