Past-due access, reliable checkout.completed, and complete morph map

[paulocastellano]

This branch bundles three related billing/infra fixes.

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1. Keep past_due subscribers in-app instead of forcing re-subscribe

Problem: when a subscription goes past_due, Cashier's default (deactivatePastDue = true) makes subscribed() return false. EnsureAccountReady then redirects to app.subscribe, which opens a fresh Stripe Checkout and creates a second subscription. A past-due customer already has one — they just need to update their payment method.

Fix:

• Cashier::keepPastDueSubscriptionsActive() — past_due counts as active again, so the middleware lets the user navigate (no forced /subscribe).
• Account::isPastDue() (false when self-hosted, else subscription(...)?->pastDue()).
• Shared Inertia prop auth.subscriptionPastDue + TS type.
• Sidebar-footer notice (red) with a CTA to the Stripe billing portal (app.billing.portal) — never /subscribe.
• i18n (en, es, pt-BR).

Both trial modes verified, neither affected (past_due only exists on a real subscription): card-required → subscription trial (subscribed() already true); no-card → generic trial (isOnTrial()/onGenericTrial()).

2. Fire checkout.completed reliably for trial-with-card checkouts

Problem (confirmed in PostHog): 66 real subscription.created vs only 3 checkout.completed. With trial-with-card the subscription is trialing (already subscribed()) by the time the webhook lands, so /billing/processing usually mounts already-active and the false→true poll transition the event depended on never happens.

Fix:

• Complete the purchase from whichever path runs first — onMounted (already active) or the poll transition.
• De-duplicated per checkout session via a one-time Cache::add gate on session_id (new fromCheckout prop), so back-button/refresh to the success URL can't re-fire.

3. Register all models in the morph map

• Added the five automation models to Relation::enforceMorphMap().
• Documented the "every model must be mapped" rule in CLAUDE.md.
• tests/Unit/MorphMapTest.php fails if any app/Models class is missing from the map.

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Tests

• BillingControllerTest, TrialMiddlewareAccessTest, AccountTest, MorphMapTest all green; broader billing/account/workspace/usage suites pass — no regressions. Pint clean.
• Note: no JS test runner in the project, so the Processing.vue timing logic is covered by reasoning + the backend fromCheckout one-time test, not a component test.