refactor(B1): migrate to domain-driven project structure

Move all models/schemas/services/routers into app/domains/.
Keep backward-compat shims in old locations for imports.
Preserves domains/rendering/tasks.py from Phase A.

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

backend/app/domains/notifications/models.py

@@ -0,0 +1,28 @@
+import uuid
+from datetime import datetime
+from sqlalchemy import String, Boolean, DateTime, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+from app.database import Base
+
+
+class AuditLog(Base):
+    __tablename__ = "audit_log"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    user_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=True)
+    action: Mapped[str] = mapped_column(String(100), nullable=False)
+    entity_type: Mapped[str] = mapped_column(String(100), nullable=True)
+    entity_id: Mapped[str] = mapped_column(String(255), nullable=True)
+    details: Mapped[dict] = mapped_column(JSONB, nullable=True)
+    timestamp: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+
+    # Notification center columns
+    target_user_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True,
+    )
+    read_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
+    notification: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
+
+    user: Mapped["User"] = relationship("User", back_populates="audit_logs", foreign_keys=[user_id])
+    target_user: Mapped["User"] = relationship("User", foreign_keys=[target_user_id])