Add Bandcamp/YouTube links on all recs, Fix My Playlist, configurable rec count

- Bandcamp: match artist+song, fall back to artist-only page
- YouTube fallback: if not on Bandcamp, link to YouTube music video search
- Fix My Playlist: AI analyzes playlist, finds outliers, suggests replacements
- User chooses recommendation count (5, 10, 15, 20)

backend/app/models/recommendation.py

@@ -21,6 +21,7 @@ class Recommendation(Base):
     preview_url: Mapped[str | None] = mapped_column(String(500), nullable=True)
     image_url: Mapped[str | None] = mapped_column(String(500), nullable=True)
     bandcamp_url: Mapped[str | None] = mapped_column(String(500), nullable=True)
+    youtube_url: Mapped[str | None] = mapped_column(String(500), nullable=True)
 
     # AI explanation
     reason: Mapped[str] = mapped_column(Text)