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]

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):

pipeline/lib/spotify-genre.py

@@ -25,8 +25,10 @@ Usage:
   spotify-genre.py --apply --force          # overwrite existing GENRE tags
   spotify-genre.py --apply --query '...'    # subset by beets query
 """
-import sys, os, re, json, argparse, base64, urllib.parse, urllib.request, subprocess
+import sys, os, re, json, argparse, urllib.parse, urllib.request, subprocess
 from pathlib import Path
+
+from _spotify_auth import get_token
 from mutagen import File as MFile
 from mutagen.id3 import ID3, ID3NoHeaderError, TCON
 from mutagen.flac import FLAC
@@ -73,19 +75,8 @@ def load_whitelist():
     return out
 
 
-_token = None
 def _spotify_token():
-    global _token
-    if _token: return _token
-    creds = base64.b64encode(f"{SPOTIFY_CID}:{SPOTIFY_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:
-        _token = json.loads(r.read())["access_token"]
-    return _token
+    return get_token(SPOTIFY_CID, SPOTIFY_CSEC)
 
 
 # Cache: artist_name → list[str] of Spotify genres (or None if not found)

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: