feat: implement AI-curated playlist service and dashboard integration

- Added hierarchical AGENTS.md knowledge base
- Implemented PlaylistService with 6h themed and 24h devotion mix logic
- Integrated AI theme generation for 6h playlists via Gemini/OpenAI
- Added /playlists/refresh and metadata endpoints to API
- Updated background worker with scheduled playlist curation
- Created frontend PlaylistsSection, Tooltip components and integrated into Dashboard
- Added Alembic migration for playlist tracking columns
- Fixed Docker healthcheck with curl installation

backend/Dockerfile

@@ -5,6 +5,7 @@ WORKDIR /app
 
 # Install system dependencies
 RUN apt-get update && apt-get install -y --no-install-recommends \
+    curl \
     && rm -rf /var/lib/apt/lists/*
 
 COPY requirements.txt .