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>

models.py

@@ -224,6 +224,11 @@ class DcaSettlement(BaseModel):
     error_message: Optional[str]
     processed_at: Optional[datetime]
     created_at: datetime
+    # Append-only audit memo. Populated when an operator triggers an in-place
+    # adjustment (partial-dispense, manual reconciliation override). Each
+    # entry timestamped + records original values so the overwrite is
+    # auditable from the settlement detail view alone. Never edited in place.
+    notes: Optional[str] = None
 
 
 # =============================================================================
@@ -397,6 +402,28 @@ class PartialDispenseData(BaseModel):
         return v
 
 
+class AppendSettlementNoteData(BaseModel):
+    """Operator-authored free-form note on a settlement.
+
+    Notes are prepended (newest first) to the settlement's `notes` column,
+    with a UTC timestamp and the author's user id so each entry is
+    accountable. Useful for cash-drawer reconciliation context, off-the-
+    record refund records, or any narrative an operator wants to attach
+    for future reference.
+    """
+
+    note: str
+
+    @validator("note")
+    def non_empty(cls, v):
+        v = v.strip() if isinstance(v, str) else v
+        if not v:
+            raise ValueError("note cannot be empty")
+        if len(v) > 2000:
+            raise ValueError("note too long (max 2000 chars)")
+        return v
+
+
 class SettleBalanceData(BaseModel):
     """Resolves satmachineadmin#4 — operator settles small remaining LP balance
     from their own wallet at the current exchange rate."""