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/universe.py

@@ -20,10 +20,7 @@ Four routes:
                                            held in memory for one LLM
                                            call, discarded on response.
 
-All routes require authentication (session cookie OR bearer token). The
-old endpoints in `app/routers/api.py` (`/api/portfolios/upload`,
-`/api/portfolio/{name}/summary`) remain live until step 10 of the Phase G
-plan, when they're removed alongside the table drops.
+All routes require authentication (session cookie OR bearer token).
 """
 from __future__ import annotations