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.lock

@@ -281,6 +281,7 @@ GEM
       bigdecimal
       rexml
     crass (1.0.6)
+    csv (3.3.5)
     date (3.5.1)
     debug (1.11.1)
       irb (~> 1.10)
@@ -720,6 +721,7 @@ DEPENDENCIES
   bundler-audit
   capybara
   console1984!
+  csv
   debug
   faker
   fizzy-saas!