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

backend/services/subscription/preflight_validator.py

@@ -241,7 +241,8 @@ def validate_exa_research_operations(
 def validate_image_generation_operations(
     pricing_service: PricingService,
     user_id: str,
-    num_images: int = 1
+    num_images: int = 1,
+    provider_name: Optional[str] = None,
 ) -> None:
     """
     Validate image generation operation(s) before making API calls.
@@ -250,25 +251,36 @@ def validate_image_generation_operations(
         pricing_service: PricingService instance
         user_id: User ID for subscription checking
         num_images: Number of images to generate (for multiple variations)
+        provider_name: Actual image provider (e.g., 'stability', 'gemini', 'huggingface', 'wavespeed')
         
     Returns:
         None
         If validation fails, raises HTTPException with 429 status
     """
     try:
+        # Map actual provider name to the APIProvider used for limit enforcement
+        provider_map = {
+            'stability': APIProvider.STABILITY,
+            'gemini': APIProvider.GEMINI,
+            'huggingface': APIProvider.MISTRAL,  # HF images track to total_calls, enforce via MISTRAL
+            'wavespeed': APIProvider.WAVESPEED,
+        }
+        api_provider = provider_map.get(provider_name or '', APIProvider.STABILITY)
+        display_name = provider_name or 'stability'
+
         # Create validation operations for each image
         operations_to_validate = [
             {
-                'provider': APIProvider.STABILITY,
+                'provider': api_provider,
                 'tokens_requested': 0,
-                'actual_provider_name': 'stability',
+                'actual_provider_name': display_name,
                 'operation_type': 'image_generation'
             }
             for _ in range(num_images)
         ]
         
-        logger.info(f"[Pre-flight Validator] 🚀 Validating {num_images} image generation(s) for user {user_id}")
-        
+        logger.info(f"[Pre-flight Validator] 🚀 Validating {num_images} image generation(s) for user {user_id}, provider={display_name}")
+
         can_proceed, message, error_details = pricing_service.check_comprehensive_limits(
             user_id=user_id,
             operations=operations_to_validate
@@ -278,7 +290,7 @@ def validate_image_generation_operations(
             logger.error(f"[Pre-flight Validator] Image generation blocked for user {user_id}: {message}")
             
             usage_info = error_details.get('usage_info', {}) if error_details else {}
-            provider = usage_info.get('provider', 'stability') if usage_info else 'stability'
+            provider = usage_info.get('provider', display_name) if usage_info else display_name
             
             raise HTTPException(
                 status_code=429,

backend/services/subscription/pricing_service.py

@@ -564,11 +564,11 @@ class PricingService:
                 "serper_calls_limit": 10,
                 "metaphor_calls_limit": 0,  # DISABLED: Metaphor not in Free tier
                 "firecrawl_calls_limit": 0,  # DISABLED: Firecrawl not in Free tier
-                "stability_calls_limit": 3,  # 3 images - enough to try the product
+                "stability_calls_limit": 10,  # 10 images - enough for 2 podcasts (5 images each)
                 "exa_calls_limit": 10,  # 10 research queries - enough to try the product
                 "video_calls_limit": 2,  # 2 video renders - try podcast video on Free
                 "image_edit_calls_limit": 5,  # 5 image edits - enough to try the product
-                "audio_calls_limit": 5,  # 5 audio clips - enough to try the product
+                "audio_calls_limit": 10,  # 10 audio clips - enough for 2 podcasts (5 clips each)
                 "wavespeed_calls_limit": 0,  # 0 = unlimited for Free; video controlled via video_calls_limit
                 "gemini_tokens_limit": 50000,
                 "openai_tokens_limit": 0,  # DISABLED