R7: consolidate duplicated Spotify-token and NDJSON-parse logic

- The client-credentials token fetch was copy-pasted across spotify-retag.py,
  spotify-genre.py, and fix-track-metadata.py (and the app's spotify_client).
  Add pipeline/lib/_spotify_auth.get_token (cached per id/secret); the three
  scripts now source their own credentials but delegate the request to it. The
  scripts run with pipeline/lib on sys.path, so the plain `from _spotify_auth
  import get_token` resolves.
- The identical _parse_json_lines helper in dedup_review_service and
  genre_review_service is now a single app/services/_ndjson.parse_json_lines.

Verified: unit test of the token helper (cache + request), the NDJSON parser
(tests/test_ndjson.py), full suite green (40), and a live spotify-genre dry-run
that fetched a token and queried Spotify through the shared helper.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pipeline/lib/spotify-retag.py

@@ -31,27 +31,15 @@ Matching strategy (per file):
      use it. If multiple, score each by artist-string overlap and pick the
      best. If nothing close, leave the file alone.
 """
-import sys, os, re, json, base64, urllib.request, urllib.parse, subprocess
+import sys, os, re, json, urllib.request, urllib.parse, subprocess
 from pathlib import Path
 from difflib import SequenceMatcher
 
+from _spotify_auth import get_token
+
 API = "https://api.spotify.com/v1"
 
 
-def get_token(cid: str, csec: str) -> str:
-    creds = base64.b64encode(f"{cid}:{csec}".encode()).decode()
-    req = urllib.request.Request(
-        "https://accounts.spotify.com/api/token",
-        data=urllib.parse.urlencode({"grant_type": "client_credentials"}).encode(),
-        headers={
-            "Authorization": f"Basic {creds}",
-            "Content-Type": "application/x-www-form-urlencoded",
-        },
-    )
-    with urllib.request.urlopen(req, timeout=15) as r:
-        return json.loads(r.read())["access_token"]
-
-
 def fetch_playlist(url: str, token: str) -> list[dict]:
     m = re.search(r"playlist[/:]([A-Za-z0-9]+)", url)
     if not m: