feat: enforce credit limits via Stripe alerts for all billing scenarios (#16801)

## Summary

This PR ensures usage alerts are created for all billing scenarios.

## Background

Per [Stripe
documentation](https://docs.stripe.com/billing/subscriptions/usage-based/alerts),
usage alerts are **one-time per customer** - they trigger once and only
consider usage reported after the alert is created. This means we need
to create a new alert whenever:

1. ✅ Subscription is created (trial) - Already implemented
2. ✅ Trial ends (user becomes Active subscriber) - **Added in this PR**
3. ✅ Credit tier changes (upgrade) - **Added in this PR**
4. ✅ **New billing cycle starts** - **Critical fix in this PR!**

## The Issue

Previously, the invoice webhook reset `hasReachedCurrentPeriodCap =
false` at cycle end, but didn't create a new alert. This meant after the
first billing period, there was no alert to trigger and users could
exceed their limit without being blocked.

## Changes

### 1. Invoice Webhook (`billing-webhook-invoice.service.ts`)
When invoice is finalized for `subscription_cycle`:
- Reset `hasReachedCurrentPeriodCap = false` ✅ (already done)
- **NEW**: Create alert at the current tier cap

### 2. End Trial Period (`billing-subscription.service.ts`)
In `endTrialPeriod()`:
- **NEW**: Create alert at the paid tier cap (not trial cap)

### 3. Credit Tier Upgrades (`billing-subscription.service.ts`)
In `changeMeteredPrice()`, when upgrading immediately (not scheduled for
period end):
- **NEW**: Reset `hasReachedCurrentPeriodCap = false`
- **NEW**: Create alert at new tier cap

### 4. Alert Title Update (`stripe-billing-alert.service.ts`)
Changed from "Trial usage cap" to "Usage cap" since alerts are now used
for all scenarios.

## Stripe Alert Limits

- Max 25 alerts per meter+customer combination
- Alerts only evaluate usage reported after creation
- One-time alerts trigger once per customer

With monthly billing cycles + occasional tier changes, we should stay
well under the 25 alert limit.

## Testing

- TypeScript typechecking passes
- Backend services properly inject new dependencies

packages/twenty-server/src/engine/core-modules/billing-webhook/services/billing-webhook-invoice.service.ts

@@ -2,11 +2,12 @@ import { Injectable, Logger } from '@nestjs/common';
 import { InjectRepository } from '@nestjs/typeorm';
 
 import { isDefined } from 'twenty-shared/utils';
-import { Repository } from 'typeorm';
+import { type Repository } from 'typeorm';
 
 import type Stripe from 'stripe';
 
 import { BillingSubscriptionItemEntity } from 'src/engine/core-modules/billing/entities/billing-subscription-item.entity';
+import { BillingSubscriptionService } from 'src/engine/core-modules/billing/services/billing-subscription.service';
 
 const SUBSCRIPTION_CYCLE_BILLING_REASON = 'subscription_cycle';
 
@@ -16,12 +17,18 @@ export class BillingWebhookInvoiceService {
   constructor(
     @InjectRepository(BillingSubscriptionItemEntity)
     private readonly billingSubscriptionItemRepository: Repository<BillingSubscriptionItemEntity>,
+    private readonly billingSubscriptionService: BillingSubscriptionService,
   ) {}
 
   async processStripeEvent(data: Stripe.InvoiceFinalizedEvent.Data) {
-    const { billing_reason: billingReason, subscription } = data.object;
+    const {
+      billing_reason: billingReason,
+      subscription,
+      customer,
+    } = data.object;
 
     const stripeSubscriptionId = subscription as string | undefined;
+    const stripeCustomerId = customer as string | undefined;
 
     if (
       isDefined(stripeSubscriptionId) &&
@@ -31,6 +38,12 @@ export class BillingWebhookInvoiceService {
         { stripeSubscriptionId },
         { hasReachedCurrentPeriodCap: false },
       );
+
+      if (isDefined(stripeCustomerId)) {
+        await this.billingSubscriptionService.createBillingAlertForCustomer(
+          stripeCustomerId,
+        );
+      }
     }
   }
 }