Add rate limit mail interceptor with Redis-backed quota tracking

[Copilot]

Adds daily and monthly email sending quotas enforced via an ActionMailer interceptor, with quota state stored in Redis and an event hook for subscribers.

Configuration

Two new ENV vars control the limits (either or both can be set):

• ACTION_MAILER_MAX_EMAILS_PER_DAY
• ACTION_MAILER_MAX_EMAILS_PER_MONTH

The interceptor is only registered when at least one limit is configured.

New components

• RateLimitMailInterceptor — checks Redis quota before each delivery; blocks the email (perform_deliveries = false), logs a WARN, and publishes an ActiveSupport::Notifications event when a limit is hit:

  ActiveSupport::Notifications.subscribe("decidim.pokecode.mail_quota_exceeded") do |*, payload|
    # payload: { period: :daily | :monthly, current_count:, limit:, recipients: }
  end

• MailQuota — Redis model wrapping the quota counters. Keys are scoped by date (decidim:pokecode:mail_quota:daily:YYYY-MM-DD) and month (…:monthly:YYYY-MM) with EXPIREAT set to end-of-period so they self-clean. Exposes daily_count, monthly_count, and increment! (atomic via MULTI). Uses a class-level shared Redis connection from REDIS_URL.

Behavior

Daily limit is checked first; if clear, monthly is checked. Quota is only incremented when delivery proceeds.