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/api/routes/admin.py

@@ -27,7 +27,6 @@ _CONFIG_FIELDS = (
     "bet_amount_sats",
     "round_duration_seconds",
     "round_cooldown_seconds",
-    "min_amount_sats",
     "fee_rate_sat_vb",
     "rbf_timeout_seconds",
     "draw_animation_seconds",
@@ -40,7 +39,6 @@ class RoundConfigResponse(BaseModel):
     bet_amount_sats: int
     round_duration_seconds: int
     round_cooldown_seconds: int
-    min_amount_sats: int
     fee_rate_sat_vb: int
     rbf_timeout_seconds: int
     draw_animation_seconds: int
@@ -52,7 +50,6 @@ class RoundConfigUpdate(BaseModel):
     bet_amount_sats: int | None = None
     round_duration_seconds: int | None = None
     round_cooldown_seconds: int | None = None
-    min_amount_sats: int | None = None
     fee_rate_sat_vb: int | None = None
     rbf_timeout_seconds: int | None = None
     draw_animation_seconds: int | None = None