feat: validate podcast cost estimation accuracy, document per-token costs, and fix subscription/plan enforcement

Issue #543 — Validate Estimated Cost Accuracy (UI vs Backend)

Backend:
- cost_estimator.py uses pricing catalog (APIProviderPricing) as single source of truth
- All 7 cost components: analysis, research (search+LLM), script, TTS, voice clone, avatar, video
- initialize_default_pricing() runs on every app startup for auto-sync

Frontend cost estimation fixes:
- Added missing analysisCost, scriptCost, voiceCloneCost to PodcastEstimate type
- toPodcastEstimate() now extracts all 7 backend fields (was dropping 3)
- headerCostEst maps analysisCost->Analyze, scriptCost->Write, voiceCloneCost->Produce
- EstimateCard shows 5 chips: Analysis, Research, Script, Voice(TTS+clone), Visuals(avatar+video)
- Chip sum now equals backend total for all configurations

Subscription & plan fixes:
- Removed Stripe re-verification from checkSubscription() (downgrade regression fix #539)
- Added verifyCheckoutRef pattern for reliable mount-time checkout polling
- One-time Stripe sync effect with pending_subscription_change flag for Customer Portal returns
- Free plan limits: stability_calls 3->10, audio_calls 5->10 (supports 2 podcasts)
- Image enforcement uses actual provider (GPT_PROVIDER), not hardcoded Stability
- Billing/pricing pages bypass onboarding check in ProtectedRoute
- Gradient buttons + loading spinner on plan chip in UserBadge
- Added metadata-based Stripe lookup fallback (Issue #538)

Documentation:
- TESTING_GUIDE.md: comprehensive testing instructions for non-technical testers
  - Free plan limits, usage tracking, cost estimation formulas
  - 10 test cases for UI verification
  - Troubleshooting guide
  - Quick-reference cost formulas with all default rates

Cleanup: removed legacy ToBeMigrated directory (70+ files, ~22K LOC)
GSC Brainstorm: service, hook, modal, and UI components for blog topic brainstorming

docs-site/docs/features/blog-writer/overview.md

@@ -16,6 +16,13 @@ The ALwrity Blog Writer is a powerful AI-driven content creation tool that helps
 - **Content Optimization**: AI-powered content improvement suggestions
 - **SEO Integration**: Built-in SEO analysis and recommendations
 
+### 🧠 Smart Topic Brainstorming
+- **[GSC Brainstorm](gsc-brainstorm-service.md)** *(NEW)*: AI-powered topic suggestions from your Google Search Console data
+- **Quick Wins Detection**: Identify easy-to-rank content opportunities
+- **Keyword Gap Analysis**: Find content gaps that could boost rankings
+- **AI Recommendations**: Get LLM-generated blog post strategies
+- **Health Score**: Track your SEO performance with a 0-100 composite score
+
 ### 🎯 User-Friendly Features
 - **Visual Editor**: Easy-to-use WYSIWYG editor with markdown support
 - **Progress Tracking**: Real-time progress monitoring for long tasks