0.2.39: bitcoin-denominated view, first-login flow, unfunded + tax center

- BTC prices: btc_prices table, CSV upload on Import page (auto-detected
  date/close columns, upsert by date), entities.close_date as the BTC entry
  mark; statements carry btc_price_cents (as-of) + btc_close_price_cents.
  LP capital blocks show paid-in vs current value in bitcoin terms.
- First login: accounts on the shared default password are flagged
  (must_change_password) and blocked behind a full-screen password change;
  external accounts then get a one-time welcome tour with a 2FA offer
  (users.onboarded_at).
- LP portal: Unfunded (callable commitment) metric; Tax documents center
  aggregating K-1/tax docs across funds, grouped by year.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/ten31portal/routers/capital_import_router.py

@@ -325,6 +325,8 @@ def commit_import(
                 role=UserRole.investor,
                 login_enabled=True,
                 external_investor_id=inv.external_id,
+                # First login forces a change while they're on the shared default.
+                must_change_password=(pw == config.DEFAULT_INVESTOR_PASSWORD),
             )
             session.add(user)
             session.flush()