fix: strip ?si= from Spotify URL, improve spotdl error logging

spotdl doesn't handle tracking params in URLs. Also separate stdout/stderr
in Python script and always try parsing stdout before checking exit code.

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

backend/spotify_resolve.py

@@ -2,7 +2,7 @@
 """
 Resolves a Spotify URL to track metadata using spotdl (TzurSoffer fork).
 Usage: python3 spotify_resolve.py <spotify_url>
-Outputs JSON to stdout.
+Outputs JSON to stdout. Errors go to stderr for clean separation.
 """
 import sys
 import json
@@ -10,36 +10,47 @@ import os
 import tempfile
 import subprocess
 
+def eprint(*args):
+    print(*args, file=sys.stderr)
+
 def main():
     if len(sys.argv) < 2:
-        print(json.dumps({"error": "No URL provided"}))
+        eprint("No URL provided")
         sys.exit(1)
 
     url = sys.argv[1]
     tmp = tempfile.mktemp(suffix=".spotdl")
 
     try:
+        eprint(f"[spotdl] running: spotdl save {url}")
         result = subprocess.run(
             ["spotdl", "save", url, "--save-file", tmp, "--log-level", "ERROR"],
             capture_output=True, text=True, timeout=60
         )
+        eprint(f"[spotdl] exit={result.returncode}")
+        if result.stdout.strip():
+            eprint(f"[spotdl] stdout: {result.stdout.strip()[:300]}")
+        if result.stderr.strip():
+            eprint(f"[spotdl] stderr: {result.stderr.strip()[:300]}")
+
         if result.returncode != 0:
-            err = result.stderr.strip() or result.stdout.strip() or "spotdl failed"
-            print(json.dumps({"error": err}))
+            err = result.stderr.strip() or result.stdout.strip() or "spotdl exited non-zero"
+            eprint(f"[spotdl] FAILED: {err}")
             sys.exit(1)
 
         if not os.path.exists(tmp):
-            print(json.dumps({"error": "spotdl produced no output file"}))
+            eprint("[spotdl] no output file produced")
             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"}))
+            eprint("[spotdl] empty track list")
             sys.exit(1)
 
-        # Transform spotdl Song format to NovaSonic format
+        eprint(f"[spotdl] got {len(songs)} tracks")
+
         def fmt_track(s):
             artists = s.get("artists") or []
             artist_str = ", ".join(artists) if artists else s.get("artist", "")
@@ -60,7 +71,6 @@ def main():
         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",