spotify: resilient fetching, progress bars, venv launcher

- fetch() uses a retrying session (connection resets, 5xx, 429 Retry-After)
  with backoff, so a transient drop mid-fetch no longer aborts the run.
- tqdm progress bars on all paginated fetches (driven by paging `total`).
- Flow.fetch_spotify(): drop stray `self` from the @staticmethod.
- gui.sh runs the venv's streamlit from the script dir; add requirements.txt.

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

requirements.txt

@@ -0,0 +1,9 @@
+streamlit==1.58.0
+streamlit-aggrid==1.2.1.post2
+pandas==3.0.3
+Pillow==12.2.0
+mutagen==1.47.0
+rapidfuzz==3.14.5
+requests==2.34.2
+tqdm==4.68.2
+xmltodict==1.0.4