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

apps/web/app/home/[account]/members/policies/route.ts

@@ -0,0 +1,69 @@
+import { NextResponse } from 'next/server';
+
+import { z } from 'zod';
+
+import { enhanceRouteHandler } from '@kit/next/routes';
+import { getSupabaseServerClient } from '@kit/supabase/server-client';
+import {
+  createInvitationContextBuilder,
+  createInvitationsPolicyEvaluator,
+} from '@kit/team-accounts/policies';
+
+export const GET = enhanceRouteHandler(
+  async function ({ params, user }) {
+    const client = getSupabaseServerClient();
+    const { account } = z.object({ account: z.string() }).parse(params);
+
+    try {
+      // Evaluate with standard evaluator
+      const evaluator = createInvitationsPolicyEvaluator();
+      const hasPolicies = await evaluator.hasPoliciesForStage('preliminary');
+
+      if (!hasPolicies) {
+        return NextResponse.json({
+          allowed: true,
+          reasons: [],
+          metadata: {
+            policiesEvaluated: 0,
+            timestamp: new Date().toISOString(),
+            noPoliciesConfigured: true,
+          },
+        });
+      }
+
+      // Build context for policy evaluation (empty invitations for testing)
+      const contextBuilder = createInvitationContextBuilder(client);
+
+      const context = await contextBuilder.buildContext(
+        {
+          invitations: [],
+          accountSlug: account,
+        },
+        user,
+      );
+
+      // validate against policies
+      const result = await evaluator.canInvite(context, 'preliminary');
+
+      return NextResponse.json(result);
+    } catch (error) {
+      return NextResponse.json(
+        {
+          allowed: false,
+          reasons: [
+            error instanceof Error ? error.message : 'Unknown error occurred',
+          ],
+          metadata: {
+            error: true,
+            originalError:
+              error instanceof Error ? error.message : String(error),
+          },
+        },
+        { status: 500 },
+      );
+    }
+  },
+  {
+    auth: true,
+  },
+);