docs(plans): update Pro pricing to $12/mo ($10/mo annual)

Matches the Clerk Dashboard prices the operator just set:
  - Pro monthly: $12
  - Pro annual:  $10/mo equivalent ($120/yr), ~17% discount for prepay

Business ($29/mo) is unchanged at this step; it'll be renamed to
"Pro Plus" in a follow-up once the Clerk product is renamed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Sbussiso

backend/app/core/plans.py

@@ -3,7 +3,7 @@
 
 Plan slugs must match the keys defined in the Clerk Dashboard:
   - free_org  (Free)
-  - pro       (Pro — $14/mo)
+  - pro       (Pro — $12/mo, or $10/mo billed annually = $120/yr)
   - business  (Business — $29/mo; will be renamed "Pro Plus" once the
                Clerk product is renamed in the dashboard)
 """