fix(v2): use payment_hash as settlement idempotency key

The initial m005 made bitspire_event_id the UNIQUE idempotency key on
dca_settlements, but settlements arriving through LNbits' invoice
listener (the canonical path per nostr-transport-branch architecture)
don't carry a Nostr event id at the Payment level — that's the
underlying transport's concern, not exposed to extensions.

The natural unique key is payment_hash:
- every LN invoice has a globally unique payment_hash
- subscription replays / dispatcher double-fires dedup via UNIQUE
- it's always present on the Payment object the invoice_listener delivers

Reshape the dca_settlements column constraints:
- payment_hash: TEXT NOT NULL UNIQUE (was: NOT NULL + separate index)
- bitspire_event_id: TEXT (was: NOT NULL UNIQUE) — kept nullable for
  a future path where we subscribe to raw kind-21000 Nostr events
  directly, bypassing the Payment system

Also rename the CRUD helper: get_settlement_by_event_id →
get_settlement_by_payment_hash, and update create_settlement_idempotent
to dedup on payment_hash. CreateDcaSettlementData / DcaSettlement
adjust accordingly.

The schema is unshipped (v2-bitspire branch is local only) — fixing
m005 in-place is appropriate. The separate dca_telemetry path for
kind-30078/30079 events already uses (machine_id, beacon_received_at)
semantics, so the UNIQUE-by-Nostr-event-id pattern isn't needed there
either.

Caught during P1a design before subscribing to register_invoice_listener.

Refs: aiolabs/satmachineadmin#9, aiolabs/lamassu-next#44

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

migrations.py

@@ -290,20 +290,28 @@ async def m005_satmachine_v2_overhaul(db):
         "ON satoshimachine.dca_deposits (client_id, created_at DESC)"
     )
 
-    # dca_settlements — idempotency table for bitSpire kind-21000 events.
-    # CRITICAL: platform_fee_sats and operator_fee_sats are stored as absolute BIGINT
-    # (not as a derived percentage). Today this is just the contractual split. Once
-    # the v2 promotion engine ships, the two values diverge when discounts fire and
-    # this row is the only audit-grade record of who forgave what. Do not collapse
-    # them into a single commission_pct field. See plan section "Customer discounts".
+    # dca_settlements — idempotency table for bitSpire-driven settlements.
+    # The natural unique key is payment_hash (every LN invoice has a globally
+    # unique hash; subscription replays / dispatcher double-fires dedup via the
+    # UNIQUE constraint). bitspire_event_id is reserved for a future path where
+    # we subscribe to raw Nostr events directly (kind-30078/30079 ingestion
+    # uses dca_telemetry; bitspire_event_id is kept here for future bookkeeping
+    # if we ever bypass the LNbits Payment system).
+    #
+    # CRITICAL: platform_fee_sats and operator_fee_sats are stored as absolute
+    # BIGINT (not a derived percentage). Today this is just the contractual
+    # split. Once the v2 promotion engine ships, the two values diverge when
+    # discounts fire and this row is the only audit-grade record of who forgave
+    # what. Do not collapse them into a single commission_pct field. See plan
+    # section "Customer discounts".
     await db.execute(
         f"""
         CREATE TABLE satoshimachine.dca_settlements (
             id TEXT PRIMARY KEY,
             machine_id TEXT NOT NULL,
-            bitspire_event_id TEXT NOT NULL UNIQUE,
+            payment_hash TEXT NOT NULL UNIQUE,
+            bitspire_event_id TEXT,
             bitspire_txid TEXT,
-            payment_hash TEXT NOT NULL,
             gross_sats BIGINT NOT NULL,
             fiat_amount DECIMAL(10,2) NOT NULL,
             fiat_code TEXT NOT NULL DEFAULT 'GTQ',
@@ -327,10 +335,7 @@ async def m005_satmachine_v2_overhaul(db):
         "CREATE INDEX dca_settlements_machine_idx "
         "ON satoshimachine.dca_settlements (machine_id, created_at DESC)"
     )
-    await db.execute(
-        "CREATE INDEX dca_settlements_payment_hash_idx "
-        "ON satoshimachine.dca_settlements (payment_hash)"
-    )
+    # payment_hash UNIQUE already creates a lookup index — no extra index needed.
 
     # dca_commission_splits — operator's rules for distributing the *remainder*
     # of each commission (commission_sats - platform_fee_sats). One row per leg.