feat(billing): add sales_leads and stripe_events tables

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/models/stripe_event.py

@@ -0,0 +1,17 @@
+from datetime import datetime, timezone
+from sqlalchemy import String, DateTime, Index
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.dialects.postgresql import JSONB
+from app.core.database import Base
+
+
+class StripeEvent(Base):
+    __tablename__ = "stripe_events"
+    __table_args__ = (Index("ix_stripe_events_event_type", "event_type"),)
+
+    id: Mapped[str] = mapped_column(String(255), primary_key=True)  # Stripe event id
+    event_type: Mapped[str] = mapped_column(String(100), nullable=False)
+    processed_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+    payload_excerpt: Mapped[dict] = mapped_column(JSONB, nullable=False, default=dict)