feat: per-tier feature toggles for recipe variations/pairings (v0.50.0)

Admins can now disable specific AI features per tier from Admin > Tier
Limits — new feature_flags table (feature x tier -> enabled, defaulting
to true so adding a new gated feature never needs a backfill).

Covers recipe variations, drink pairing, and meal pairing to start.
When disabled for a user's tier, the button stays visible (with a small
lock badge) but opens an upgrade dialog instead of running; the API
route rejects the call server-side either way (requireFeatureEnabled,
re-reads tier from the DB rather than trusting the session's cache,
same rationale as checkAndIncrementTierLimit).

The upgrade dialog is informational only — no Stripe checkout exists
yet (STRIPE_PLAN.md is still just a plan) — its CTA links to /support
prefilled as an upgrade-interest suggestion.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/admin/tiers/page.tsx

@@ -1,11 +1,16 @@
 import type { Metadata } from "next";
 import { db, tierDefinitions } from "@epicure/db";
 import { TierLimitsForm } from "@/components/admin/tier-limits-form";
+import { FeatureFlagsForm } from "@/components/admin/feature-flags-form";
+import { getFeatureFlagMatrix, FEATURE_DEFINITIONS } from "@/lib/feature-flags";
 
 export const metadata: Metadata = {};
 
 export default async function AdminTiersPage() {
-  const tiers = await db.select().from(tierDefinitions);
+  const [tiers, featureFlagMatrix] = await Promise.all([
+    db.select().from(tierDefinitions),
+    getFeatureFlagMatrix(),
+  ]);
 
   return (
     <div className="space-y-8">
@@ -19,6 +24,11 @@ export default async function AdminTiersPage() {
       {tiers.map((tierDefinition) => (
         <TierLimitsForm key={tierDefinition.tier} tierDefinition={tierDefinition} />
       ))}
+
+      <FeatureFlagsForm
+        features={FEATURE_DEFINITIONS.map((f) => ({ key: f.key, label: f.label, description: f.description }))}
+        initialMatrix={featureFlagMatrix}
+      />
     </div>
   );
 }