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_auth.py

@@ -0,0 +1,35 @@
+"""Shared Spotify Client-Credentials token fetch for the pipeline scripts.
+
+The scripts (spotify-retag.py, spotify-genre.py, fix-track-metadata.py) each
+run as `python /app/pipeline/lib/<name>.py`, so this directory is on sys.path
+and they can `from _spotify_auth import get_token`. Each still sources its own
+credentials (env vars vs a rendered .env file); only the token request itself,
+which was copy-pasted four ways, lives here.
+"""
+import base64
+import json
+import urllib.parse
+import urllib.request
+
+_TOKEN_URL = "https://accounts.spotify.com/api/token"
+_cache: dict[tuple[str, str], str] = {}
+
+
+def get_token(client_id: str, client_secret: str) -> str:
+    """Return a client-credentials access token, cached per (id, secret) for
+    the life of the process."""
+    key = (client_id, client_secret)
+    if key in _cache:
+        return _cache[key]
+    creds = base64.b64encode(f"{client_id}:{client_secret}".encode()).decode()
+    req = urllib.request.Request(
+        _TOKEN_URL,
+        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:
+        _cache[key] = json.loads(r.read())["access_token"]
+    return _cache[key]