feat: add AI suggestion audit trail table

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/ai_suggestion.py

@@ -0,0 +1,55 @@
+"""AI Suggestion model for tracking AI-applied changes to flows."""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional
+
+from sqlalchemy import DateTime, ForeignKey, String
+from sqlalchemy.dialects.postgresql import JSONB, UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.database import Base
+
+
+class AISuggestion(Base):
+    __tablename__ = "ai_suggestions"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    tree_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("trees.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    session_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("ai_chat_sessions.id", ondelete="SET NULL"),
+        nullable=True,
+    )
+    action_type: Mapped[str] = mapped_column(
+        String(50), nullable=False
+    )
+    target_node_id: Mapped[Optional[str]] = mapped_column(
+        String(255), nullable=True
+    )
+    changes_json: Mapped[dict] = mapped_column(
+        JSONB, nullable=False, default=dict
+    )
+    status: Mapped[str] = mapped_column(
+        String(20), nullable=False, default="pending"
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        nullable=False,
+    )
+    resolved_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )