feat(knex): create database interfaces and knexjs nest integration

Co-authored-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Erik Michelson <github@erik.michelson.eu>

backend/src/database/identity.ts

@@ -0,0 +1,32 @@
+/*
+ * SPDX-FileCopyrightText: 2025 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { ProviderType } from '../auth/provider-type.enum';
+
+/**
+ * An auth identity holds the information how a {@link User} can authenticate themself using a certain auth provider
+ */
+export interface Identity {
+  /** The id of the user */
+  userId: number;
+
+  /** The type of the auth provider */
+  providerType: ProviderType;
+
+  /** The identifier of the auth provider, e.g. gitlab */
+  providerIdentifier: string | null;
+
+  /** Timestamp when this identity was created */
+  createdAt: Date;
+
+  /** Timestamp when this identity was last updated */
+  updatedAt: Date;
+
+  /** The remote id of the user at the auth provider or null for local identities */
+  providerUserId: string | null;
+
+  /** The hashed password for local identities or null for other auth providers */
+  passwordHash: string | null;
+}