Feature Policies API + Invitations Policies (#375)

- Added Feature Policy API: a declarative system to enable/disable/modify default behavior in the SaaS kit
- Team invitation policies with pre-checks using the Feature Policy API: Invite Members dialog now shows loading, errors, and clear reasons when invitations are blocked
- Version bump to 2.16.0 and widespread dependency updates (Supabase, React types, react-i18next, etc.).
- Added comprehensive docs for the new policy system and orchestrators.
- Subscription cancellations now trigger immediate invoicing explicitly

packages/features/team-accounts/src/server/policies/policies.ts

@@ -0,0 +1,70 @@
+import { allow, definePolicy, deny } from '@kit/policies';
+import { createPolicyRegistry } from '@kit/policies';
+
+import { FeaturePolicyInvitationContext } from './feature-policy-invitation-context';
+
+/**
+ * Feature-specific registry for invitation policies
+ */
+export const invitationPolicyRegistry = createPolicyRegistry();
+
+/**
+ * Subscription required policy
+ * Checks if the account has an active subscription
+ */
+export const subscriptionRequiredInvitationsPolicy =
+  definePolicy<FeaturePolicyInvitationContext>({
+    id: 'subscription-required',
+    stages: ['preliminary', 'submission'],
+    evaluate: async ({ subscription }) => {
+      if (!subscription || !subscription.active) {
+        return deny({
+          code: 'SUBSCRIPTION_REQUIRED',
+          message: 'teams:policyErrors.subscriptionRequired',
+          remediation: 'teams:policyRemediation.subscriptionRequired',
+        });
+      }
+
+      return allow();
+    },
+  });
+
+/**
+ * Paddle billing policy
+ * Checks if the account has a paddle subscription and is in a trial period
+ */
+export const paddleBillingInvitationsPolicy =
+  definePolicy<FeaturePolicyInvitationContext>({
+    id: 'paddle-billing',
+    stages: ['preliminary', 'submission'],
+    evaluate: async ({ subscription }) => {
+      // combine with subscriptionRequiredPolicy if subscription must be required
+      if (!subscription) {
+        return allow();
+      }
+
+      // Paddle specific constraint: cannot update subscription items during trial
+      if (
+        subscription.provider === 'paddle' &&
+        subscription.status === 'trialing'
+      ) {
+        const hasPerSeatItems = subscription.items.some(
+          (item) => item.type === 'per_seat',
+        );
+
+        if (hasPerSeatItems) {
+          return deny({
+            code: 'PADDLE_TRIAL_RESTRICTION',
+            message: 'teams:policyErrors.paddleTrialRestriction',
+            remediation: 'teams:policyRemediation.paddleTrialRestriction',
+          });
+        }
+      }
+
+      return allow();
+    },
+  });
+
+// register policies below to apply them
+//
+//