feat: use spotdl (TzurSoffer fork) as primary Spotify resolver

Replaces rate-limited api.spotify.com calls with spotdl's SpotAPI
approach, which uses Spotify's internal browser endpoints — no
developer credentials needed, no api.spotify.com rate limits.
TOTP anonymous token kept as automatic fallback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/spotify_resolve.py

@@ -0,0 +1,99 @@
+#!/usr/bin/env python3
+"""
+Resolves a Spotify URL to track metadata using spotdl (TzurSoffer fork).
+Usage: python3 spotify_resolve.py <spotify_url>
+Outputs JSON to stdout.
+"""
+import sys
+import json
+import os
+import tempfile
+import subprocess
+
+def main():
+    if len(sys.argv) < 2:
+        print(json.dumps({"error": "No URL provided"}))
+        sys.exit(1)
+
+    url = sys.argv[1]
+    tmp = tempfile.mktemp(suffix=".spotdl")
+
+    try:
+        result = subprocess.run(
+            ["spotdl", "save", url, "--save-file", tmp, "--log-level", "ERROR"],
+            capture_output=True, text=True, timeout=60
+        )
+        if result.returncode != 0:
+            err = result.stderr.strip() or result.stdout.strip() or "spotdl failed"
+            print(json.dumps({"error": err}))
+            sys.exit(1)
+
+        if not os.path.exists(tmp):
+            print(json.dumps({"error": "spotdl produced no output file"}))
+            sys.exit(1)
+
+        with open(tmp, "r", encoding="utf-8") as f:
+            songs = json.load(f)
+
+        if not songs:
+            print(json.dumps({"error": "No tracks found"}))
+            sys.exit(1)
+
+        # Transform spotdl Song format to NovaSonic format
+        def fmt_track(s):
+            artists = s.get("artists") or []
+            artist_str = ", ".join(artists) if artists else s.get("artist", "")
+            date = s.get("date") or (str(s["year"]) if s.get("year") else "")
+            return {
+                "trackName":         s.get("name", ""),
+                "artistName":        artist_str,
+                "collectionName":    s.get("album_name", ""),
+                "artworkUrl100":     s.get("cover_url", ""),
+                "releaseDate":       date,
+                "trackNumber":       s.get("track_number"),
+                "trackExplicitness": "explicit" if s.get("explicit") else "notExplicit",
+                "durationMs":        int(s["duration"] * 1000) if s.get("duration") else None,
+            }
+
+        tracks = [fmt_track(s) for s in songs]
+        first = songs[0]
+        artists0 = first.get("artists") or []
+        artist0 = ", ".join(artists0) if artists0 else first.get("artist", "")
+
+        # Determine type from URL
+        if "/track/" in url:
+            out = {
+                "type":       "track",
+                "name":       first.get("name", ""),
+                "artist":     artist0,
+                "artworkUrl": first.get("cover_url", ""),
+                "tracks":     tracks,
+            }
+        elif "/album/" in url:
+            out = {
+                "type":       "album",
+                "name":       first.get("album_name", ""),
+                "artist":     first.get("album_artist", artist0),
+                "artworkUrl": first.get("cover_url", ""),
+                "tracks":     tracks,
+            }
+        else:
+            out = {
+                "type":        "playlist",
+                "name":        first.get("list_name", ""),
+                "artist":      first.get("album_artist", ""),
+                "artworkUrl":  first.get("cover_url", ""),
+                "skippedLocal": 0,
+                "tracks":      tracks,
+            }
+
+        print(json.dumps(out, ensure_ascii=False))
+
+    finally:
+        try:
+            os.unlink(tmp)
+        except Exception:
+            pass
+
+if __name__ == "__main__":
+    main()