fix(v2): decouple listener + skipped-leg audit (fix bundle 2)

Closes H4, H5, M8 from the v2-bitspire review (omnibus follow-up #11).

H4 — Decouple invoice listener from distribution.
  tasks._handle_payment now spawns process_settlement on a background
  task instead of awaiting it. The LNbits invoice queue is shared
  across every extension on the node; under load (a machine with 50
  LPs, a stalled internal payment, etc.) the previous synchronous path
  could freeze the queue for everyone. Concurrency is safe because
  fix bundle 1's claim_settlement_for_processing already prevents
  double-processing on listener re-fires.

  RUF006 fix: hold strong refs to in-flight tasks via a module-level
  set so the GC doesn't collect them mid-flight (asyncio.create_task
  only weakly references its task). Tasks self-clean via
  add_done_callback(set.discard).

H5 + M8 — Skipped-leg audit rows for stranded sats.
  Previously, four paths in distribution.py logged a warning and left
  sats in the machine wallet, marking the settlement 'processed' with
  no row-level visibility into where the un-paid sats sit:
    1. _pay_super_fee: super_fee_pct > 0 but super_fee_wallet_id unset
    2. _pay_operator_splits: no commission ruleset (default + override)
    3. _pay_dca_distributions: exchange_rate = 0 (fallback path)
    4. _pay_dca_distributions: no eligible LPs with positive balance
  Plus a fifth case the review didn't enumerate but is the same shape:
    5. _pay_dca_distributions: no flow-mode LPs at the machine at all

  Each now writes a dca_payments row with status='skipped', the
  intended leg_type (super_fee / operator_split / dca), the stranded
  amount in amount_sats, and a human-readable error_message explaining
  why. New _record_skipped_leg helper consolidates the pattern.

  This makes stranded sats visible in:
    - The machine detail dialog's settlements rows (the legs are
      filtered into the audit blob alongside completed/failed legs)
    - The payments CSV export
    - GET /api/v1/dca/payments?leg_type=...

  'skipped' is a documented leg-status value now (alongside pending /
  completed / failed / voided / refunded) — no schema change since
  status is TEXT.

Knock-on fix: void_open_legs_for_settlement (used by partial-dispense
recompute) now also includes status='skipped' in its WHERE clause so a
re-run doesn't double-count the audit rows from a prior attempt.

72/72 tests still pass. Lint clean.

Refs: aiolabs/satmachineadmin#11 — fix bundle 2 ✅
Remaining in #11: H6 (partial-dispense split ratio) + fix bundle 3
(dead-code purge) + the M and N items.

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

models.py

@@ -323,7 +323,16 @@ class DcaPayment(BaseModel):
     exchange_rate: Optional[float]
     transaction_time: datetime
     external_payment_hash: Optional[str]
-    status: str  # 'pending' | 'completed' | 'failed' | 'refunded'
+    status: str
+    # Leg status enum:
+    #   'pending'   — row written, payment not yet attempted
+    #   'completed' — pay_invoice succeeded; sats moved
+    #   'failed'    — pay_invoice errored; sats stayed at source
+    #   'voided'    — superseded (e.g. partial-dispense recompute voided
+    #                 the previous pending/failed leg)
+    #   'skipped'   — intentionally not paid (no super wallet configured,
+    #                 no commission ruleset, no exchange rate, no LPs)
+    #   'refunded'  — reserved for future refund flows
     error_message: Optional[str]
     created_at: datetime