v0.8.7: update VERSION, README, and code comments

- VERSION bumped to 0.8.7
- README.md: comprehensive update with Gokapi integration, stems radio
  buttons, download button, cost tracking improvements, stepper fixes,
  error recovery, new env vars, new troubleshooting entries
- app.py: updated module docstring with all current routes
- helpers.py: updated module docstring with function group overview,
  added section header for Gokapi integration
- config.py: added Gokapi env vars to docstring
- models.py: updated schema comments for per-version costs and
  stems_link/stems_url distinction

helpers.py

@@ -5,6 +5,15 @@ Utility and configuration helpers for the Theme Song Booth Flask app.
 
 These functions are stateless (or use Flask's current_app / session context)
 and are imported by app.py. Keeping them here reduces the size of the route file.
+
+Key function groups:
+  - Genre/decade parsing and style formatting
+  - Admin auth, settings persistence, encryption helpers
+  - Email sending (SMTP) and ntfy notifications
+  - Hermes API key management and prompt callback signing
+  - MusicGPT API client: generation, polling, file downloads, cost tracking
+  - Gokapi integration: stems upload, zip bundling, share link generation
+  - MP3 metadata tagging (mutagen)
 """
 
 import os
@@ -797,6 +806,10 @@ def get_musicgpt_cost_totals():
     }
 
 
+# ---------------------------------------------------------------------------
+# Gokapi file-sharing integration (stems upload)
+# ---------------------------------------------------------------------------
+
 def parse_stems_urls(stems_url_field):
     """
     Parse the stems_url DB field into a list of (label, url) tuples.