fix: validate settings that crash at runtime instead of load time

[rcbevans]

Summary

Nine WorkerSettings fields accepted values that passed load-time validation but crashed at runtime. Each now fails fast at settings load with a clear error message.

Fields fixed

Field	Problem	Fix
log_level	Accepted any string	Validator against {DEBUG, INFO, WARNING, ERROR, CRITICAL} (case-insensitive, normalized to uppercase)
sso_backend	Accepted any string	Validator against {none, oidc, saml} (case-insensitive, normalized to lowercase)
poll_interval	No lower bound — 0 or negative crashes asyncio.sleep	ge=0.1
notify_health_check_interval	No lower bound	ge=0.1
notify_reconnect_backoff_initial	No lower bound	ge=0.01
prune_schedule_utc	Accepted any string despite "HH:MM" description	Regex + range validator (00–23 hours, 00–59 minutes)
archive_expiry_schedule_utc	Same	Reuses _hh_mm_validator (parameterized via ctx.field_name)
prune_cron_expr	Accepted any string when set	croniter.is_valid() in a validator hook (skips None/empty default)
archive_expiry_cron_expr	Same	Reuses _cron_expr_validator

Implementation

Follows existing patterns in settings.py:

• Validator hooks (_log_level_validator, _sso_backend_validator, _hh_mm_validator, _cron_expr_validator) match the _log_format_validator / _VALID_LOG_FORMATS pattern — raising ValueError which dotenvmodel wraps as ConstraintViolationError.
• Field constraints (ge=0.1, ge=0.01) match existing ge= usage on other float fields.
• croniter is already a core dependency, imported at module level.
• The _hh_mm_validator and _cron_expr_validator are reusable — ctx.field_name produces the correct field name in each error message.

Verification

uv run ruff check .                          # All checks passed
uv run ruff format --check .                 # 475 files already formatted
uv run pyright src/taskq/settings.py         # 0 errors, 0 warnings
uv run pytest tests/test_settings.py -v      # 127 passed in 0.79s

33 new tests cover valid values, invalid values, boundary conditions, and case-insensitivity for each field.