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

models.py

@@ -10,8 +10,11 @@ Schema overview (see SCHEMA constant):
 - requests table stores customer data, generated prompts, file paths,
   approval state, email timestamps, payment reference, player token,
   vocal gender preference, revision count, revision note, and operator notes.
-- MusicGPT integration fields store task/conversion IDs, status, cost, error,
-  album cover URL, WAV paths, and stems task/cost/URL.
+- MusicGPT integration fields store task/conversion IDs, status, per-version
+  costs (musicgpt_cost_a/b accumulated into musicgpt_cost), error, album cover
+  URL, WAV paths, and stems task/cost/URL.
+- Stems integration: stems_url holds raw CDN URLs from the webhook, stems_link
+  holds the Gokapi share link (auto-generated on completion or manually uploaded).
 - Revisions: when a customer requests changes, the current A/B MP3 files are
   renamed to archived "RevN-" copies and new versions are uploaded later.
 - operator_notes is an internal column for the booth team and is never shown
@@ -54,7 +57,7 @@ CREATE TABLE IF NOT EXISTS requests (
     revision_count INTEGER DEFAULT 0,
     revision_note TEXT,
     operator_notes TEXT,
-    stems_link TEXT,
+    stems_link TEXT,                -- Gokapi share link for stems (auto-generated, also editable by operator)
     stems_interest INTEGER DEFAULT 0,
     musicgpt_task_id TEXT,           -- MusicGPT Music AI task ID shared by Version A and B
     musicgpt_conversion_id_1 TEXT,   -- conversion ID for Version A
@@ -71,7 +74,7 @@ CREATE TABLE IF NOT EXISTS requests (
     stems_task_id TEXT,              -- MusicGPT Extraction task ID for optional stems
     stems_status TEXT,               -- status of the Extraction job
     stems_cost REAL,                 -- API-reported stems cost in USD credits
-    stems_url TEXT,                  -- download / share link for stems
+    stems_url TEXT,                  -- raw stem URLs from MusicGPT webhook (label: url; label: url)
     stems_error TEXT                 -- error message from stems job
 );