[RFC] Consolidate users in a single table

Idea

Have a single trusted table for users.
It should contain:

• id: Snowflake ID, can be used later as foreign key
• login: current uid
• email: user primary email address
• status: enabled/disabled
• source: provider who created the user (Database, LDAP…), maybe with a last_sync
• last_seen: timestamp of last ping on Nextcloud (not updated on all requests)
• last_login: last successful connection on Nextcloud

Why

It's often difficult to find information about users because it needs to fetch information from several backends.
Examples:

• is this user exists? We have to check in all backends
• is this user disabled? It's stored as a preference
• get a list of user who actually connect? Also a preference
• search a user? We need to search in all backends

This table should be a single source of truth for users.

The related class can cache users so it becomes easier to know if a user exists or not.

Challenge

External backends like LDAP can add and remove user at any time. Our table must stay in sync with the backends.
This can be a background job but we have to find a good frequency.

It will introduce a latency between user added/deleted from backend and user actually in Nextcloud database.
The class should probably introduce a fallback for some important operations like login.

Start by mapping the existing user backends, preference-based status and activity data, and the login path described in the issue. Define the table's ownership and synchronization behavior, including external additions and removals, background-job frequency, and login fallback before implementation. Done means the design resolves these trade-offs and identifies the affected components and validation approach.