fix(api/security): restrict history settings read to admins

[Micsi]

Motivation

• Prevent disclosure of sensitive backend credentials (InfluxDB tokens/username/password and TimescaleDB DSN) that were returned by GET /api/v1/system/history/settings to any authenticated user.
• Ensure consistency with the existing admin-only PUT /history/settings and POST /history/test routes.

Description

• Require admin authorization by changing the dependency from Depends(get_current_user) to Depends(get_admin_user) on the GET /history/settings route in obs/api/v1/system.py and update the route docstring to indicate admin-only access.
• A related route dependency was also tightened in obs/api/v1/system.py to use get_admin_user where present (keeps behavior limited to privileged callers).
• Kept the response model and payload unchanged for authorized admin callers to preserve existing admin functionality.

Testing

• Ran python -m py_compile obs/api/v1/system.py which completed successfully.
• Attempted pytest -q tests/integration/test_history.py tests/integration/test_auth.py, which could not run in this environment due to a missing test dependency (pytest_asyncio).

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Codex Task