feat(v2): commission split target accepts wallet id, invoice key, LN address, or LNURL

Closes the operator-facing limitation that commission split legs could
only target the operator's own LNbits wallets. Adopting the splitpayments
pattern — a single `target` string accepts:
  - LNbits wallet id (UUID-shaped) — direct internal pay
  - LNbits wallet invoice key — resolved via get_wallet_for_key, then
    internal pay (lets the operator split to any LNbits user who shares
    their invoice key)
  - Lightning address (user@domain) — resolved via LNURL-pay
  - LNURL string (LNURL1...) — resolved via LNURL-pay

Schema (m001 update — fresh-install only; no operator data in production):
  dca_commission_splits.wallet_id → target

Backend (distribution.py):
  - New _pay_split_leg helper: routes the leg by target type. External
    targets (@ or LNURL prefix) go through get_pr_from_lnurl + pay_invoice;
    internal targets go through create_invoice + pay_invoice (the original
    path), with get_wallet_for_key as the first resolution step so
    invoice keys work as well as wallet ids.
  - _pay_operator_splits delegates per-leg payment to the new helper.
  - dca_payments rows still record the leg as leg_type='operator_split';
    external targets land destination_ln_address (the human-readable
    target), internal targets land destination_wallet_id.
  - Errors are caught and surfaced via the existing failed-leg path
    so /retry can re-run them.

Frontend (commission tab):
  - Each leg gets a per-row q-btn-toggle: "My wallet" vs "Lightning
    address / LNURL / invoice key". Wallet mode shows the q-select of
    the operator's own wallets (previous behaviour); external mode
    shows a free-text q-input.
  - On load, targetKind is inferred from whether the stored target
    matches one of the operator's wallet ids (renders as 'wallet')
    or not (renders as 'external'). The kind is UI-only, not persisted.
  - Leg row laid out in a bordered card so the toggle + 3-column layout
    don't crowd at narrow widths.

Refs: aiolabs/satmachineadmin#9

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

models.py

@@ -245,13 +245,27 @@ class DcaSettlement(BaseModel):
 
 
 class CommissionSplitLeg(BaseModel):
-    """Single leg of an operator's commission-split rule set."""
+    """Single leg of an operator's commission-split rule set.
 
-    wallet_id: str
+    `target` accepts any of (splitpayments pattern):
+      - LNbits wallet id
+      - LNbits wallet invoice key (resolved server-side via get_wallet_for_key)
+      - Lightning address (user@domain)
+      - LNURL string (bech32 LNURL...)
+    """
+
+    target: str
     label: Optional[str] = None
     pct: float
     sort_order: int = 0
 
+    @validator("target")
+    def non_empty_target(cls, v):
+        v = (v or "").strip()
+        if not v:
+            raise ValueError("target cannot be empty")
+        return v
+
     @validator("pct")
     def pct_in_unit_range(cls, v):
         if v < 0 or v > 1:
@@ -263,7 +277,7 @@ class CommissionSplit(BaseModel):
     id: str
     machine_id: Optional[str]  # None = operator's default ruleset
     operator_user_id: str
-    wallet_id: str
+    target: str
     label: Optional[str]
     pct: float
     sort_order: int