feat(v2): partial-dispense + operator notes on settlements (P3d)

Closes the v1 feature request satmachineadmin#3 (partial transaction
processing) and adds operator-authored audit notes on settlements.

Schema (m006_add_settlement_notes):
  ALTER TABLE dca_settlements ADD COLUMN notes TEXT

The notes column is append-only (prepend with timestamp, never edit in
place). Stores both system-generated audit memos (partial-dispense
recompute provenance) and operator-authored free-form notes (cash-
drawer reconciliation context, off-LN refund records, etc.).

Partial-dispense endpoint:
  POST /api/v1/dca/settlements/{id}/partial-dispense
  body: PartialDispenseData {dispensed_fraction OR dispensed_sats, notes}

Recompute path (in distribution.apply_partial_dispense_and_redistribute):
  1. Refuse if any leg has status='completed' (Lightning can't claw back)
  2. Resolve new_gross from dispensed_fraction or dispensed_sats
  3. Linear-scale net/commission/fiat — preserves the original commission
     ratio exactly; only rounding may drift by 1 sat
  4. Re-stage-1 split using the CURRENT super_fee_pct (super may have
     changed the rate since the original landed)
  5. Build a memo capturing original values + reason + new values
  6. Void pending/failed legs (status → 'voided')
  7. Overwrite the settlement's monetary fields + prepend memo to notes
  8. Reset status to 'pending' → process_settlement re-runs distribution

Operator notes endpoint:
  POST /api/v1/dca/settlements/{id}/notes
  body: AppendSettlementNoteData {note}

Each operator note is timestamped (UTC) and tagged with the author's
user_id so the audit trail is accountable. Non-empty, max 2000 chars.

72/72 tests still pass. 30 routes total. The full-directory ruff number
ballooned to ~500 because it includes legacy transaction_processor.py
(orphaned, not imported anywhere) and other v1 cruft on the branch.
Files I actively maintain are clean.

Note: a richer queryable audit history (filter by author / time range /
action type / etc.) is being tracked as a separate future-work issue.
The notes-column approach here is the v1 audit story; the dedicated
history table will be additive.

Refs: aiolabs/satmachineadmin#9, closes #3 (in spirit, marked
once verified end-to-end)

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

migrations.py

@@ -428,3 +428,21 @@ async def m005_satmachine_v2_overhaul(db):
         );
         """
     )
+
+
+async def m006_add_settlement_notes(db):
+    """Audit memo on dca_settlements.
+
+    When an operator triggers an in-place adjustment (partial-dispense,
+    manual reconciliation override, etc.), the settlement row's monetary
+    fields are overwritten with the new numbers. To preserve the audit
+    trail without a separate history table, we append a timestamped memo
+    to this notes column capturing the previous values and the reason.
+
+    Operators see this directly in the settlement detail view, so any
+    overwrite is visible and dated. Append-only convention: new memos
+    are prepended with a timestamp; never edited in place.
+    """
+    await db.execute(
+        "ALTER TABLE satoshimachine.dca_settlements ADD COLUMN notes TEXT"
+    )