Three issues combined to make the per-user 'Allowed models' checklist
unreliable (#3032):
1. admin.js _loadModelsForUser fetched /api/models, which is backed by
cached_models — endpoints that haven't been probed yet (e.g. a
freshly-added DeepSeek API endpoint) simply didn't show up in the
checklist. Switched to /api/model-endpoints, which always reflects
every configured endpoint regardless of cache state.
2. _saveModels sent allowed_models: [] both when the admin clicked
[All] (no restriction) and [None] (block everything) — the backend
had no way to distinguish the two.
3. _enforce_chat_privileges treated an empty allowed_models list as
'no restriction' (falsy -> skip the check), so [None] had no effect.
Added an explicit block_all_models privilege flag (defaulting to False,
and forced to False for admins) that admin.js now sets when zero models
are checked. _enforce_chat_privileges checks it first and 403s
regardless of allowed_models contents.
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