Add UI-configurable InfrastructureSettings with DB+env fallback for all features

Add 5 NotebookLM parity features: Mind Map, Infographic, Slide Deck, Deep Research, Video Generator

feat(podcasts): model registry integration, credential passthrough & new features (#632)

fix: turn the embedding field into optional (#557)

feat: credential-based API key management (#477) (#540)

feat: content-type aware chunking and unified embedding (#444)

perf: improve source listing speed by 20-30x (#436)

refactor: reorganize folder structure for better maintainability