Add Last.fm import support

Users can import their top tracks from Last.fm by entering their username.
No OAuth required - uses the public Last.fm API with user.getTopTracks and
user.getInfo endpoints. Includes a preview feature to verify the username
and see sample tracks before importing. Supports configurable time periods
(all time, 7 days, 1/3/6/12 months). Free tier playlist limit enforced.

backend/app/services/lastfm.py

@@ -0,0 +1,100 @@
+import httpx
+
+from app.core.config import settings
+
+LASTFM_API_URL = "http://ws.audioscrobbler.com/2.0/"
+
+
+async def get_user_info(username: str) -> dict | None:
+    """Verify a Last.fm username exists and return basic info."""
+    params = {
+        "method": "user.getInfo",
+        "user": username,
+        "api_key": settings.LASTFM_API_KEY,
+        "format": "json",
+    }
+    async with httpx.AsyncClient(timeout=15) as client:
+        resp = await client.get(LASTFM_API_URL, params=params)
+
+    if resp.status_code != 200:
+        return None
+
+    data = resp.json()
+    if "error" in data:
+        return None
+
+    user = data.get("user", {})
+    image_url = None
+    images = user.get("image", [])
+    for img in images:
+        if img.get("size") == "large" and img.get("#text"):
+            image_url = img["#text"]
+
+    return {
+        "name": user.get("name", username),
+        "display_name": user.get("realname") or user.get("name", username),
+        "playcount": int(user.get("playcount", 0)),
+        "image_url": image_url,
+        "url": user.get("url", ""),
+    }
+
+
+async def get_top_tracks(
+    username: str, period: str = "overall", limit: int = 50
+) -> list[dict]:
+    """Fetch a user's top tracks from Last.fm.
+
+    Args:
+        username: Last.fm username.
+        period: Time period - overall, 7day, 1month, 3month, 6month, 12month.
+        limit: Max number of tracks to return (max 1000).
+
+    Returns:
+        List of dicts with: title, artist, playcount, image_url.
+    """
+    valid_periods = {"overall", "7day", "1month", "3month", "6month", "12month"}
+    if period not in valid_periods:
+        period = "overall"
+
+    params = {
+        "method": "user.getTopTracks",
+        "user": username,
+        "period": period,
+        "limit": min(limit, 1000),
+        "api_key": settings.LASTFM_API_KEY,
+        "format": "json",
+    }
+    async with httpx.AsyncClient(timeout=30) as client:
+        resp = await client.get(LASTFM_API_URL, params=params)
+
+    if resp.status_code != 200:
+        raise ValueError(f"Last.fm API returned status {resp.status_code}")
+
+    data = resp.json()
+    if "error" in data:
+        raise ValueError(data.get("message", "Last.fm API error"))
+
+    raw_tracks = data.get("toptracks", {}).get("track", [])
+    tracks = []
+    for t in raw_tracks:
+        image_url = None
+        images = t.get("image", [])
+        for img in images:
+            if img.get("size") == "large" and img.get("#text"):
+                image_url = img["#text"]
+
+        artist_name = ""
+        artist = t.get("artist")
+        if isinstance(artist, dict):
+            artist_name = artist.get("name", "")
+        elif isinstance(artist, str):
+            artist_name = artist
+
+        tracks.append({
+            "title": t.get("name", "Unknown"),
+            "artist": artist_name or "Unknown",
+            "playcount": int(t.get("playcount", 0)),
+            "image_url": image_url,
+        })
+
+    return tracks