Remove payment/subscription system and card/storage limits

Fizzy is now free. Remove the entire Stripe billing system,
subscription management, and card/storage limit enforcement.

Removes from saas/: Plan model, Account::Billing, Account::Subscription,
Account::Limited, Account::OverriddenLimits, Account::BillingWaiver,
all subscription/billing controllers and views, Stripe webhook handler,
card creation/publishing limit enforcement, admin account override UI,
usage report rake task, and all related tests.

Removes from main app: Fizzy.saas? guards for subscription panel,
SaaS card footer override, near-limit notices, and saas.css stylesheet.

Adds migration to drop billing tables from the SaaS database.

Non-billing SaaS features (push notifications, signup, authorization,
telemetry, console1984/audits1984) are preserved.

Gemfile.saas.lock

@@ -281,7 +281,6 @@ GEM
       bigdecimal
       rexml
     crass (1.0.6)
-    csv (3.3.5)
     date (3.5.1)
     debug (1.11.1)
       irb (~> 1.10)
@@ -632,7 +631,6 @@ GEM
     stimulus-rails (1.3.4)
       railties (>= 6.0.0)
     stringio (3.2.0)
-    stripe (18.0.1)
     thor (1.5.0)
     thruster (0.1.18)
     thruster (0.1.18-aarch64-linux)
@@ -721,7 +719,6 @@ DEPENDENCIES
   bundler-audit
   capybara
   console1984!
-  csv
   debug
   faker
   fizzy-saas!
@@ -757,7 +754,6 @@ DEPENDENCIES
   sqlite3 (>= 2.0)
   stackprof
   stimulus-rails
-  stripe (~> 18.0)
   thruster
   trilogy (~> 2.10)
   turbo-rails!