fix(billing): no plan changes after cancellation, no unknown terms

A cancelled subscription still reported an upgrade as allowed and priced it, so
a caller trusting that would have provisioned and billed a customer who had
already left. And any term string other than the two we support was silently
priced monthly while keeping the unknown value — a subscription whose price and
billing period disagree. Both are refused now.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Models/Subscription.php

@@ -86,6 +86,12 @@ class Subscription extends Model
             throw new RuntimeException("Unknown plan: {$plan}");
         }
 
+        // A typo would otherwise be priced monthly while carrying an unknown
+        // term — a subscription whose price and billing period disagree.
+        if (! in_array($term, [self::TERM_MONTHLY, self::TERM_YEARLY], true)) {
+            throw new RuntimeException("Unknown term: {$term}");
+        }
+
         $monthly = (int) ($catalogue['price_cents'] ?? 0);
 
         return [

app/Services/Billing/PlanChange.php

@@ -36,6 +36,22 @@ final readonly class PlanChange
     public static function evaluate(Subscription $subscription, string $targetPlan, ?Carbon $at = null): self
     {
         $at = $at?->copy() ?? now();
+
+        // A cancelled contract has nothing left to change. Without this, a
+        // caller trusting allowedNow would provision and bill an upgrade for a
+        // customer who has already left.
+        if ($subscription->status !== 'active') {
+            return new self(
+                isUpgrade: false,
+                allowedNow: false,
+                chargeCents: 0,
+                creditCents: 0,
+                effectiveAt: null,
+                remainingDays: 0,
+                termDays: 1,
+            );
+        }
+
         $target = Subscription::snapshotFrom($targetPlan, $subscription->term);
 
         // Direction comes from the plan's rank, never from the prices: a

tests/Feature/Billing/PlanChangeTest.php

@@ -208,3 +208,23 @@ it('sends an upgrade requested after the term into the next one', function () {
         ->and($change->chargeCents)->toBe(0)
         ->and($change->effectiveAt->toDateString())->toBe('2026-09-01');
 });
+
+it('has nothing to change once a subscription is cancelled', function () {
+    $subscription = Subscription::factory()->plan('start')->create([
+        'status' => 'cancelled',
+        'cancelled_at' => now(),
+    ]);
+
+    // A caller trusting allowedNow would otherwise provision and bill an
+    // upgrade for a customer who has already left.
+    $change = PlanChange::evaluate($subscription, 'business');
+
+    expect($change->allowedNow)->toBeFalse()
+        ->and($change->chargeCents)->toBe(0)
+        ->and($change->effectiveAt)->toBeNull();
+});
+
+it('refuses a term it does not know', function () {
+    expect(fn () => Subscription::snapshotFrom('team', 'yearlyy'))
+        ->toThrow(RuntimeException::class, 'Unknown term');
+});