M34: enforce domain integrity in PostgreSQL

backend/app/models/outbox.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import datetime
 
-from sqlalchemy import DateTime, Integer, String, Text
+from sqlalchemy import CheckConstraint, DateTime, Index, Integer, String, Text
 from sqlalchemy.dialects.postgresql import JSONB, UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -32,6 +32,14 @@ def is_demo_scenario_failure(event: "OutboxEvent") -> bool:
 
 class OutboxEvent(TimestampMixin, Base):
     __tablename__ = "outbox_events"
+    __table_args__ = (
+        CheckConstraint(
+            "delivery_status IN ('pending','delivering','succeeded','failed')",
+            name="ck_outbox_delivery_status",
+        ),
+        CheckConstraint("attempts >= 0", name="ck_outbox_attempts"),
+        Index("ix_outbox_delivery_next_attempt", "delivery_status", "next_attempt_at"),
+    )
 
     event_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), primary_key=True, default=uuid.uuid4