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/fix-track-metadata.py

@@ -36,6 +36,8 @@ Supported URLs:
 """
 import sys, os, re, json, base64, argparse, subprocess, unicodedata, urllib.parse, urllib.request
 
+from _spotify_auth import get_token
+
 from mutagen import File as MFile
 from mutagen.id3 import ID3, ID3NoHeaderError, TPE1, TPE2, TALB, TIT2, TRCK, TDRC, TCON, APIC
 from mutagen.flac import FLAC, Picture
@@ -87,14 +89,7 @@ def _spotify_token():
                 continue
             k, v = line.split("=", 1)
             env[k] = v.strip().strip("'").strip('"')
-    cid, csec = env["SPOTIFY_CLIENT_ID"], env["SPOTIFY_CLIENT_SECRET"]
-    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"})
-    return json.loads(urllib.request.urlopen(req, timeout=15).read())["access_token"]
+    return get_token(env["SPOTIFY_CLIENT_ID"], env["SPOTIFY_CLIENT_SECRET"])
 
 
 def _spotify_get(path, token):