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

frontend/src/components/PodcastMaker/PodcastDashboard.tsx

@@ -90,13 +90,21 @@ const PodcastDashboard: React.FC = () => {
     }
 
     if (estimate) {
+      const analyzeCost = breakdownMap.get("Analyze") || 0;
       const gatherCost = breakdownMap.get("Gather") || 0;
+      const writeCost = breakdownMap.get("Write") || 0;
       const produceCost = breakdownMap.get("Produce") || 0;
+      if (analyzeCost === 0 && estimate.analysisCost > 0) {
+        breakdownMap.set("Analyze", estimate.analysisCost);
+      }
       if (gatherCost === 0 && estimate.researchCost > 0) {
         breakdownMap.set("Gather", estimate.researchCost);
       }
+      if (writeCost === 0 && estimate.scriptCost > 0) {
+        breakdownMap.set("Write", estimate.scriptCost);
+      }
       if (produceCost === 0) {
-        breakdownMap.set("Produce", estimate.ttsCost + estimate.avatarCost + estimate.videoCost);
+        breakdownMap.set("Produce", estimate.ttsCost + estimate.voiceCloneCost + estimate.avatarCost + estimate.videoCost);
       }
     }