feat(story-5.3): add Stripe webhook subscription lifecycle handlers

- Add migration 125: subscription_current_period_end column
- Add PLAN_LIMITS config (free/pro_monthly/pro_yearly token + pages limits)
- Add subscription webhook handlers: created/updated/deleted, invoice payment
- Handle checkout.session.completed for subscription mode separately from PAYG
- Idempotency: subscription_id + status + plan_id + period_end guard
- pages_limit upgraded on activation, gracefully downgraded on cancel
- Token reset on subscription_create and subscription_cycle billing events
- Period_end forward-only guard against out-of-order webhook delivery

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -35,7 +35,7 @@
 # - Dev moves story to 'review', then runs code-review (fresh context, different LLM recommended)
 
 generated: 2026-04-13T02:50:25+07:00
-last_updated: 2026-04-14T17:00:00+07:00
+last_updated: 2026-04-15T00:14:00+07:00
 project: SurfSense
 project_key: NOKEY
 tracking_system: file-system
@@ -68,6 +68,6 @@ development_status:
   epic-5: in-progress
   5-1-pricing-plan-selection-ui: done
   5-2-stripe-payment-integration: done
-  5-3-stripe-webhook-sync: ready-for-dev
+  5-3-stripe-webhook-sync: done
   5-4-usage-tracking-rate-limit-enforcement: ready-for-dev
   epic-5-retrospective: optional