feat: bootstrap backend MVP and architecture docs

backend/app/repositories/users_repo.py

@@ -0,0 +1,38 @@
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from app.models.user import User
+
+
+class UsersRepository:
+    def __init__(self, db: Session) -> None:
+        self.db = db
+
+    def get_by_sub(self, authentik_sub: str) -> User | None:
+        stmt = select(User).where(User.authentik_sub == authentik_sub)
+        return self.db.scalar(stmt)
+
+    def upsert_by_sub(
+        self,
+        authentik_sub: str,
+        email: str | None,
+        display_name: str | None,
+        is_active: bool,
+    ) -> User:
+        user = self.get_by_sub(authentik_sub)
+        if user is None:
+            user = User(
+                authentik_sub=authentik_sub,
+                email=email,
+                display_name=display_name,
+                is_active=is_active,
+            )
+            self.db.add(user)
+        else:
+            user.email = email
+            user.display_name = display_name
+            user.is_active = is_active
+
+        self.db.commit()
+        self.db.refresh(user)
+        return user