Tie the withdrawal minimum to the bet amount instead of a separate field

RoundConfig.min_amount_sats was an independently-configurable floor that
could drift out of sync with bet_amount_sats for no real reason (deposits
never had a server-side minimum anyway). Drop the field and enforce
amount_sats >= config.bet_amount_sats directly in request_withdrawal.

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

app/db/models.py

@@ -75,7 +75,7 @@ class RoundConfig(Base):
     """Single-row operational config, DB-backed so it's editable without a redeploy.
 
     Everything business/round-related lives here (round timing, bet amount, fee
-    rate, RBF timeout, minimum amount) so an operator can tune it live. Secrets
+    rate, RBF timeout) so an operator can tune it live. Secrets
     and infra wiring (master key, JWT secret, Electrum host, admin token,
     database URL) deliberately stay env-var-driven — those require a restart
     anyway and aren't safe to hot-swap."""
@@ -92,7 +92,6 @@ class RoundConfig(Base):
     # NOT gate the actual draw, which still waits for a real confirmed block for
     # its entropy (rounds/scheduler.py) — that can take longer than this value.
     draw_animation_seconds: Mapped[int] = mapped_column(default=20)
-    min_amount_sats: Mapped[int] = mapped_column(BigInteger, default=100_000_000)
     fee_rate_sat_vb: Mapped[int] = mapped_column(default=1)
     rbf_timeout_seconds: Mapped[int] = mapped_column(default=900)
     # Maintenance switch: when true, the round currently in progress still runs to