Add playlist export and SEO meta tags

Add text/CSV export endpoints for playlists and saved recommendations.
Add export buttons to PlaylistDetail and Recommendations pages.
Add Open Graph and Twitter meta tags to index.html for better SEO.

backend/app/api/endpoints/playlists.py

@@ -1,4 +1,5 @@
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, Query
+from fastapi.responses import PlainTextResponse
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
@@ -49,6 +50,39 @@ async def get_playlist(
     return playlist
 
 
+@router.get("/{playlist_id}/export")
+async def export_playlist(
+    playlist_id: int,
+    format: str = Query("text", pattern="^(text|csv)$"),
+    user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    result = await db.execute(
+        select(Playlist).options(selectinload(Playlist.tracks))
+        .where(Playlist.id == playlist_id, Playlist.user_id == user.id)
+    )
+    playlist = result.scalar_one_or_none()
+    if not playlist:
+        raise HTTPException(status_code=404, detail="Playlist not found")
+
+    if format == "csv":
+        lines = ["Title,Artist,Album"]
+        for t in playlist.tracks:
+            # Escape commas in CSV
+            title = f'"{t.title}"' if ',' in t.title else t.title
+            artist = f'"{t.artist}"' if ',' in t.artist else t.artist
+            album = f'"{t.album}"' if t.album and ',' in t.album else (t.album or '')
+            lines.append(f"{title},{artist},{album}")
+        return PlainTextResponse("\n".join(lines), media_type="text/csv",
+            headers={"Content-Disposition": f'attachment; filename="{playlist.name}.csv"'})
+    else:
+        lines = [f"{playlist.name}", "=" * len(playlist.name), ""]
+        for i, t in enumerate(playlist.tracks, 1):
+            lines.append(f"{i}. {t.artist} - {t.title}")
+        return PlainTextResponse("\n".join(lines), media_type="text/plain",
+            headers={"Content-Disposition": f'attachment; filename="{playlist.name}.txt"'})
+
+
 @router.delete("/{playlist_id}")
 async def delete_playlist(
     playlist_id: int,