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/app/models.py

@@ -118,3 +118,15 @@ class AnalysisSnapshot(Base):
     narrative_report = Column(JSON)  # The output from the LLM (NarrativeService output)
 
     model_used = Column(String, nullable=True)  # e.g. "gemini-1.5-flash"
+    playlist_theme = Column(
+        String, nullable=True
+    )  # AI-generated theme name (e.g., "Morning Focus Mode")
+    playlist_theme_reasoning = Column(
+        Text, nullable=True
+    )  # AI explanation for why this theme
+    six_hour_playlist_id = Column(
+        String, nullable=True
+    )  # Spotify playlist ID for 6-hour playlist
+    daily_playlist_id = Column(
+        String, nullable=True
+    )  # Spotify playlist ID for 24-hour playlist