feat(cash-in): super_config.max_cash_in_sats per-tx cap + UI (#31)

Wires the server-side per-transaction cash-in ceiling the `create_withdraw`
handler already enforces (it read the value defensively via getattr; this
makes it a first-class config field).

- migrations.py m012: ADD COLUMN super_config.max_cash_in_sats INTEGER (NULL
  = no cap).
- models.py: SuperConfig.max_cash_in_sats + UpdateSuperConfigData field with a
  >= 0 validator.
- super-fee dialog: a "Max cash-in per transaction (sats)" input; blank sends
  null (the PUT skips null, preserving the current value — set 0 to reject
  every cash-in). crud `update_super_config` and the PUT endpoint flow the
  field through automatically (dynamic dict update; check_super_user gated).

Why a sats cap and not the bunker ACL: the ACL / usage caps (#28) gate call
*rate*, not *sats*, and `principal_sats` is necessarily ATM-attested — so a
single in-rate call could request an arbitrarily large payout. This bounds a
compromised/buggy machine to one capped transaction.

Verified on the dev stack: m012 runs, the model round-trips the column
(GET returns the set value), and a negative value is rejected.

models.py

@@ -482,6 +482,10 @@ class SuperConfig(BaseModel):
     super_cash_in_fee_fraction: float = 0.0
     super_cash_out_fee_fraction: float = 0.0
     super_fee_wallet_id: str | None
+    # Per-transaction cash-in ceiling in sats (#31). The bunker ACL gates call
+    # rate, not sats, so this bounds a single ATM-attested principal. NULL = no
+    # cap.
+    max_cash_in_sats: int | None = None
     updated_at: datetime
 
 
@@ -489,6 +493,7 @@ class UpdateSuperConfigData(BaseModel):
     super_cash_in_fee_fraction: float | None = None
     super_cash_out_fee_fraction: float | None = None
     super_fee_wallet_id: str | None = None
+    max_cash_in_sats: int | None = None
 
     @validator(
         "super_cash_in_fee_fraction",
@@ -501,6 +506,14 @@ class UpdateSuperConfigData(BaseModel):
             raise ValueError("super fee fraction must be between 0 and 1")
         return round(float(v), 4)
 
+    @validator("max_cash_in_sats")
+    def _cap_non_negative(cls, v):
+        if v is None:
+            return v
+        if v < 0:
+            raise ValueError("max_cash_in_sats must be >= 0")
+        return int(v)
+
 
 # =============================================================================
 # Operator UX action carriers — partial-tx and balance-settlement features.