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.

static/js/index.js

@@ -103,7 +103,8 @@ window.app = Vue.createApp({
         data: {
           super_cash_in_fee_fraction: 0,
           super_cash_out_fee_fraction: 0,
-          super_fee_wallet_id: ''
+          super_fee_wallet_id: '',
+          max_cash_in_sats: null
         }
       },
 
@@ -576,7 +577,8 @@ window.app = Vue.createApp({
           this.superConfig?.super_cash_in_fee_fraction ?? 0,
         super_cash_out_fee_fraction:
           this.superConfig?.super_cash_out_fee_fraction ?? 0,
-        super_fee_wallet_id: this.superConfig?.super_fee_wallet_id || ''
+        super_fee_wallet_id: this.superConfig?.super_fee_wallet_id || '',
+        max_cash_in_sats: this.superConfig?.max_cash_in_sats ?? null
       }
       this.superFeeDialog.show = true
     },
@@ -604,6 +606,21 @@ window.app = Vue.createApp({
         Number(d.super_cash_in_fee_fraction),
         Number(d.super_cash_out_fee_fraction)
       )) return
+      // Blank cap field -> null (the PUT skips null, so the existing value is
+      // preserved rather than cleared — set 0 to reject every cash-in).
+      const cap =
+        d.max_cash_in_sats === '' ||
+        d.max_cash_in_sats === null ||
+        d.max_cash_in_sats === undefined
+          ? null
+          : Number(d.max_cash_in_sats)
+      if (cap !== null && (!Number.isInteger(cap) || cap < 0)) {
+        Quasar.Notify.create({
+          type: 'negative',
+          message: 'Max cash-in must be a non-negative whole number of sats'
+        })
+        return
+      }
       this.superFeeDialog.saving = true
       try {
         const {data} = await LNbits.api.request(
@@ -611,7 +628,8 @@ window.app = Vue.createApp({
           {
             super_cash_in_fee_fraction: Number(d.super_cash_in_fee_fraction),
             super_cash_out_fee_fraction: Number(d.super_cash_out_fee_fraction),
-            super_fee_wallet_id: (d.super_fee_wallet_id || '').trim() || null
+            super_fee_wallet_id: (d.super_fee_wallet_id || '').trim() || null,
+            max_cash_in_sats: cap
           }
         )
         this.superConfig = data