chore: sync in-progress work across marketing, admin, API and tests

Snapshot of uncommitted work that had accumulated in the tree alongside
the Turnstile changes:

- marketing pages, SEO helpers (lib/seo.ts, lib/marketing/) and
  structured data
- admin billing actions and a per-user portfolio view, plus an admin
  error boundary
- rate limiting (lib/rate-limit.ts) applied across the /api/v1 surface
- CSP and proxy adjustments, accounting/webhook lib updates
- Playwright config and an e2e/unit test suite
- next bumped to ^16.3.4 with the lockfile regenerated
- generated AGENTS.md / CLAUDE.md

Authored by other sessions working in this tree; committed here so the
Turnstile work could be pushed without leaving the tree dirty.
Typecheck passes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/api/tenants/route.ts

@@ -5,8 +5,7 @@ import { tenants, units } from "@/lib/db/schema"
 import { getSessionUser } from "@/lib/session"
 import { tenantSchema } from "@/lib/validations"
 import { ownsProperty, ownsUnit } from "@/lib/db/ownership"
-import { getUserPlan } from "@/lib/plan-limits"
-import { checkLimit } from "@/lib/stripe/plans"
+import { checkTenantLimit } from "@/lib/plan-limits"
 import { logActivity } from "@/lib/activity"
 import { getEffectiveOwnerId, getAccountContext } from "@/lib/account"
 import { emitWebhookEvent } from "@/lib/webhooks/emit"
@@ -64,18 +63,8 @@ export async function POST(request: Request) {
   const parsed = tenantSchema.safeParse(body)
   if (!parsed.success) return NextResponse.json({ error: parsed.error.flatten() }, { status: 400 })
 
-  // Enforce per-plan tenant limit (Starter = 3).
-  const plan = await getUserPlan(ownerId)
-  const [{ count }] = await db
-    .select({ count: sql<number>`count(*)::int` })
-    .from(tenants)
-    .where(eq(tenants.user_id, ownerId))
-  if (!checkLimit(plan, "maxTenants", count)) {
-    return NextResponse.json(
-      { error: "Plan limit reached. Upgrade to add more tenants." },
-      { status: 403 }
-    )
-  }
+  const limitError = await checkTenantLimit(ownerId)
+  if (limitError) return NextResponse.json({ error: limitError }, { status: 403 })
 
   if (
     !(await ownsProperty(ownerId, parsed.data.property_id)) ||