feat(membership): enforce tenant subscription plans with blocked UI

Add configurable membership plans (free/plus/pro) synced to DB on boot, a
subscription guard middleware that blocks tenant endpoints on expired or
missing plans, and a MembershipBlockedView that surfaces the reason so
the admin can contact the tenant owner. Quota and brand-library reads now
honor the active plan's policy JSON and expired subscriptions.

server/internal/shared/config/config_test.go

@@ -4,6 +4,7 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+	"time"
 )
 
 func TestLoadPrefersEnvLLMAPIKey(t *testing.T) {
@@ -113,6 +114,55 @@ brand_library: {}
 	}
 }
 
+func TestLoadAppliesMembershipDefaults(t *testing.T) {
+	configPath := writeTestConfig(t, `
+membership: {}
+`)
+
+	cfg, err := Load(configPath)
+	if err != nil {
+		t.Fatalf("Load() error = %v", err)
+	}
+
+	if cfg.Membership.DefaultPlanCode != "free" {
+		t.Fatalf("expected default plan code free, got %q", cfg.Membership.DefaultPlanCode)
+	}
+	if len(cfg.Membership.Plans) != 3 {
+		t.Fatalf("expected 3 default plans, got %d", len(cfg.Membership.Plans))
+	}
+
+	free, ok := cfg.Membership.FindPlan("free")
+	if !ok {
+		t.Fatalf("expected free plan to exist")
+	}
+	if free.ArticleGeneration != 3 {
+		t.Fatalf("expected free article generation 3, got %d", free.ArticleGeneration)
+	}
+	if free.ImageStorageBytes != 10*1024*1024 {
+		t.Fatalf("expected free image storage 10MB, got %d", free.ImageStorageBytes)
+	}
+	if free.CompanyLimit != 1 {
+		t.Fatalf("expected free company limit 1, got %d", free.CompanyLimit)
+	}
+	if free.SubscriptionDuration != 72*time.Hour {
+		t.Fatalf("expected free subscription duration 72h, got %s", free.SubscriptionDuration)
+	}
+	if !free.ContactAdminOnExpiry {
+		t.Fatalf("expected free plan to require admin contact on expiry")
+	}
+
+	pro, ok := cfg.Membership.FindPlan("pro")
+	if !ok {
+		t.Fatalf("expected pro plan to exist")
+	}
+	if pro.ArticleGeneration != 400 {
+		t.Fatalf("expected pro article generation 400, got %d", pro.ArticleGeneration)
+	}
+	if pro.CompanyLimit != 2 {
+		t.Fatalf("expected pro company limit 2, got %d", pro.CompanyLimit)
+	}
+}
+
 func writeTestConfig(t *testing.T, body string) string {
 	t.Helper()