feat(podcast): add pre-estimate endpoint, enhance cost estimator with multi-model support, cleanup alpha pricing seeding

- Add POST /podcast/pre-estimate endpoint for cost estimation before analysis
- Enhance cost_estimator.py with multi-model support (gemini, audio, voice clone, image, video)
- Add detailed cost breakdown (llm, audio, media costs + per-phase breakdown)
- Remove redundant pricing seeding from init_alpha_subscription_tiers.py
- Add SSOT pricing via PricingService.initialize_default_pricing()
- Update TopicUrlInput tooltip to show estimate details
- Add debug logging for pricing seeding and pre-estimate
- Clean up verbose podcast mode debug logs in app.py

backend/models/podcast_models.py

@@ -45,6 +45,9 @@ class PodcastProject(Base):
     knobs = Column(JSON, nullable=True)  # Knobs settings
     research_provider = Column(String(50), nullable=True, default="google")  # Research provider
     
+    # Project-specific topic context (category research, selected topics)
+    topic_context = Column(JSON, nullable=True)  # { category: "news"|"finance", topics: [...], selected_topic: {...} }
+    
     # UI state
     show_script_editor = Column(Boolean, default=False)
     show_render_queue = Column(Boolean, default=False)