Add routing drain and single-account modes

[Rtx09x]

Summary

Adds dashboard-selectable routing modes for quota control:

• sequential_drain: drains lower-capacity accounts first until exhausted.
• reset_drain: drains accounts whose weekly quota reset is nearest first, while still skipping exhausted primary/weekly windows.
• single_account: routes eligible proxy traffic through one selected account configured from dashboard settings.

Also adds a nullable dashboard_settings.single_account_id field, a settings dropdown that appears when single-account routing is selected, and status/settings labels for the new modes.

Behavior

Single-account mode scopes normal proxy account selection to the configured account using the existing account selection path, so account status, quota exhaustion, API-key account scope, sticky behavior, and failover error handling remain centralized.

Validation

• uv run pytest tests/unit/test_load_balancer.py tests/integration/test_settings_api.py -q
• uv run pytest tests/integration/test_migrations.py::test_run_startup_migrations_drops_accounts_email_unique_with_non_cascade_fks -q
• uv run pytest tests/integration/test_settings_api.py -q
• uv run python -m compileall app
• npm run test -- --run src/features/settings/schemas.test.ts src/features/settings/components/routing-settings.test.tsx src/utils/constants.test.ts --pool=threads --maxWorkers=1
• npm run build
• git diff --check