feat(docs/internal): 📝 Add Redis User DB structure reference

internal/references/redis-user-database.mdx

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+---
+title: Redis-based User Database Structure
+description:
+  Overview of how we use the Redis key-value storage to store user data.
+---
+
+<!--
+Reference
+=========
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+=== When to use this template:
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+can quickly lookup a fact. These articles typically consist of a list, a
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+<!-- Relevant imports: -->
+
+import { Reference, Image } from '/components';
+
+<!-- Short description of what information this document provides: -->
+
+We use a simple set of key-value pairs to store any individual user.
+
+Any user has
+
+- an id
+- an email address
+- a role (either `'user'` or `'admin'`)
+- a password (saved as a hash)
+
+ID, email and role are accessible under `_USERS/[id]/*` and the id can be
+"reverse looked up" based on the email via `_USER_EMAILS/[email]`:
+
+<Image
+	src="/static/img/drawio-diagrams/internal/redis-user-db-structure.drawio.svg"
+	invertible
+/>
+
+This way, it's possible to perform all CRUD user operations with four Redis
+commands:
+
+- `SET [key] [value]` - set or add some values
+- `GET [key]` - get some values
+- `DEL [key]` - delete values
+- `KEYS [pattern]` - lookup all keys that match the `[pattern]`
+
+:::tip List of users
+
+The only thing that may seem difficult with this key-value based storage, at
+first, is listing all users. But we can easily achieve this using the `KEYS`
+command:
+
+```js
+const raw = await references.redisConn.client.KEYS('_USERS/*/email');
+
+const userIds = raw.map(s => s.split('/')[1]);
+```
+
+:::
+
+<!-- Content (e.g., a table, a list, a diagram, or a definition list) -->
+
+## See also
+
+<!--
+Snippets
+--------
+
+<Reference to="../other-article">
+    Relative Link to other article
+</Reference>
+
+<Reference to="https://www.example.com">
+    Example Website
+</Reference>
+-->