Switch user update DTO to Unset

## Scope

|Target|Change|
|---|---|
|DTO|`common/dto/manager/v2/user/request.py` — 9 fields|
|Adapter|`manager/api/adapters/user/adapter.py` — 9 branches; `manager/api/gql/user/resolver/mutation.py` — 8 branches|
|Tests|`tests/unit/common/dto/manager/v2/user/test_request.py` and any other `SENTINEL` references|

## Common work

- DTO: `X | Sentinel | None = Field(default=SENTINEL)` → `X | None | Unset = Field(default=UNSET)`, importing from `ai.backend.common.tristate.unset`
- Adapter: `TriState.from_unset` for nullable columns, `OptionalState.from_unset` for non-nullable columns
- Unify `Field(description=...)` wording to "Omit to leave unchanged; null clears." (drop "null clears" for non-nullable fields)
- Tests: `SENTINEL` → `UNSET`, `Sentinel` → `Unset`
- News fragment `changes/.enhance.md`

## Done when

- Zero `Sentinel` / `SENTINEL` references remain in this domain
- Verified with `./bai admin user update`: field omitted → unchanged, `null` → cleared, value → updated

JIRA Issue: BA-7764

Start with common/dto/manager/v2/user/request.py, then trace the nine adapter branches in manager/api/adapters/user/adapter.py and eight resolver branches in manager/api/gql/user/resolver/mutation.py. Update the tests in tests/unit/common/dto/manager/v2/user/test_request.py and other SENTINEL references, add changes/.enhance.md, and run ./bai admin user update to verify omitted, null, and value inputs.

Written by the indexing model from the issue text.