feat: Restrict Knowledge Base File Upload Count & Size, per User Group

[rbx]

Followup from #23848

Currently RAG_FILE_MAX_SIZE and RAG_FILE_MAX_COUNT exist, but they only apply to files uploaded in Chat - not to Knowledge Base uploads.

I'd like to add equivalent limits for Knowledge Base uploads. There are actually three levels at which limits could apply:

Global (like existing RAG_FILE_MAX_SIZE/RAG_FILE_MAX_COUNT): - enforced for everyone including admins, no per-user flexibility.
Default user permissions: the baseline for all non-admin users not assigned to any specific group, configurable via env vars.
Per-group: individual groups can override the default, allowing different quotas for different teams.

I implemented levels 2 and 3 in PR #23992:

Two new group permissions: Knowledge Max Files and Knowledge Max File Size, editable in Admin Panel -> Users -> Groups -> Permissions, both for individual groups and for the global default.
Optional env vars for the default user permission baseline: USER_PERMISSIONS_WORKSPACE_KNOWLEDGE_MAX_COUNT and USER_PERMISSIONS_WORKSPACE_KNOWLEDGE_MAX_SIZE.
Merging follows existing additive semantics: most-permissive value wins across a user's groups; 0 or empty means unlimited. Admins always bypass the limits.

Happy to adjust - for instance, starting with just a global cap (level 1) as a simpler first step.

What do you think?