feat: bootstrap backend MVP and architecture docs

backend/app/models/permission.py

@@ -0,0 +1,31 @@
+from datetime import datetime
+from uuid import uuid4
+
+from sqlalchemy import DateTime, ForeignKey, String, UniqueConstraint, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base import Base
+
+
+class Permission(Base):
+    __tablename__ = "permissions"
+    __table_args__ = (
+        UniqueConstraint(
+            "user_id",
+            "scope_type",
+            "scope_id",
+            "module",
+            "action",
+            name="uq_permissions_user_scope_module_action",
+        ),
+    )
+
+    id: Mapped[str] = mapped_column(UUID(as_uuid=False), primary_key=True, default=lambda: str(uuid4()))
+    user_id: Mapped[str] = mapped_column(UUID(as_uuid=False), ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    scope_type: Mapped[str] = mapped_column(String(32), nullable=False)
+    scope_id: Mapped[str] = mapped_column(String(128), nullable=False)
+    module: Mapped[str] = mapped_column(String(128), nullable=False)
+    action: Mapped[str] = mapped_column(String(32), nullable=False)
+
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)