cleanup: drop stale tombstones and dead config fields

Stale comments referencing completed migrations:
- universe.py "remain live until step 10 of Phase G" — endpoints gone
- api.py "Portfolio endpoints moved to universe.py" — empty block
- csv_import.py "persist_pie removed in Phase G" — historical context

Dead Settings fields (all confirmed unreferenced by app code):
- CASSANDRA_PORT — port is hardcoded in docker-compose / uvicorn cmd
- POLAR_API_KEY — Polar was replaced by Stripe
- CASSANDRA_MOCK — env var still set by tests as a sentinel; the
  Settings field itself was never read
- CASSANDRA_BASE_CURRENCY — "GBP" hardcoded inline elsewhere

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/routers/api.py

@@ -506,10 +506,6 @@ async def log_days(
     return templates.TemplateResponse(request, "partials/calendar.html", payload)
 
 
-# Portfolio endpoints moved to app/routers/universe.py (Phase G). The
-# server no longer persists per-user portfolio data; holdings live in
-# the browser's localStorage and prices come from /api/universe.
-
 
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