M39: harden application and acceptance gates

backend/app/models/customer.py

@@ -23,4 +23,4 @@ class Customer(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     merged_into_customer_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("customers.id")
     )
-    anonymized_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    anonymized_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), index=True)

backend/app/models/idempotency.py

@@ -15,5 +15,8 @@ class IdempotencyRecord(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     booking_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), ForeignKey("bookings.id"), nullable=False
     )
+    # SHA-256 of the canonical request body. Replaying a key with a *different* body is
+    # a client bug and must be rejected instead of silently answered with the old result.
+    request_fingerprint: Mapped[str | None] = mapped_column(String(64), nullable=True)
     response_status: Mapped[int] = mapped_column(Integer, nullable=False)
     response_body: Mapped[dict] = mapped_column(JSONB, nullable=False)