feat(backend): harden T5 — divergence comment + invariant tests

backend/src/cyclone/db.py

@@ -233,6 +233,12 @@ class Remittance(Base):
         String(32), ForeignKey("batches.id", ondelete="CASCADE"), nullable=False
     )
     payer_claim_control_number: Mapped[str] = mapped_column(String(64), nullable=False)
+    # ORM policy: no ON DELETE (forward FK from Remittance → Claim). A claim
+    # is unlikely to be deleted in normal flow (audit trail); if it ever is,
+    # the application layer (T7 reconciliation) must clear this FK. The
+    # migration predates this rationale; amendment deferred to post-SQLite
+    # rollout. SQLite without `PRAGMA foreign_keys=ON` does not enforce,
+    # so the divergence is benign in this engine.
     claim_id: Mapped[Optional[str]] = mapped_column(
         String(64), ForeignKey("claims.id"), nullable=True
     )