SaaS usage reporting (#2690)

* Add saas:usage_report rake task and extract Subscription.paid scope

Add a rake task to generate a CSV usage report with per-account data:
Queenbee ID, sign up date, paid date, card count, storage used, and
last active date.

Extract the paid subscriptions query from Admin::StatsController into
an Account::Subscription.paid scope so both the controller and the
new rake task can share it.

* Add comped and account name columns to usage report

* Update saas/lib/tasks/fizzy/usage_report.rake

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Preload storage_total to avoid N+1 in usage report

* Batch last_active_at queries to avoid per-account aggregates

* Add tests for Account::Subscription.paid scope

* Update saas/app/models/account/subscription.rb

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Aggregate paid dates in SQL instead of loading all subscriptions

* Fix paid scope to derive plan keys from Plan.all instead of PLANS hash

* Move paid dates and comped lookups into per-batch queries

* Materialize batch IDs to avoid cross-database subquery

SaasRecord models live on a separate database (fizzy_saas) that doesn't
have the accounts table. Using batch.select(:id) generated a subquery
that ran on the saas database, causing a table-not-found error. Using
pluck(:id) materializes the IDs into an array instead.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Gemfile.saas

@@ -10,6 +10,7 @@ gem "queenbee", bc: "queenbee-plugin"
 gem "fizzy-saas", path: "saas"
 gem "console1984", bc: "console1984"
 gem "audits1984", bc: "audits1984", branch: "flavorjones/coworker-api"
+gem "csv"
 
 # Native push notifications (iOS/Android)
 gem "action_push_native"