feat: add unlimited option to tier limits

Use -1 as a sentinel meaning "no cap" in tier_definitions numeric
columns. checkAndIncrementTierLimit skips the usage-cap WHERE clause
when the limit is -1. Admin tier UI gets an Unlimited switch per
field that disables the input and sends -1.

apps/web/app/api/v1/admin/tiers/[tier]/route.ts

@@ -2,6 +2,7 @@ import { NextRequest, NextResponse } from "next/server";
 import { requireAdmin } from "@/lib/api-auth";
 import { db, tierDefinitions, auditLogs, eq } from "@epicure/db";
 import { randomUUID } from "crypto";
+import { UNLIMITED } from "@/lib/tiers";
 
 interface RouteContext {
   params: Promise<{ tier: string }>;
@@ -25,7 +26,7 @@ export async function PATCH(req: NextRequest, { params }: RouteContext) {
   for (const field of NUMERIC_FIELDS) {
     const value = body[field];
     if (value === undefined) continue;
-    if (!Number.isInteger(value) || value < 0) {
+    if (!Number.isInteger(value) || (value < 0 && value !== UNLIMITED)) {
       return NextResponse.json({ error: `Invalid value for ${field}` }, { status: 400 });
     }
     updateData[field] = value;