Summary

Closes #1101.

• /apply (tier-manifest) and /apply-selections (raw selections) now both delegate to a single new _apply_selections_core helper in src/hal0/api/routes/installer.py — the apply_setup call and background-pull scheduling live in exactly one place. /apply is now a thin tier→selections adapter (_bundle_to_selections) over that shared core, matching the pattern /apply-selections already used.
• _apply_selections_core now calls apply_setup(..., write_sentinel=True) (previously both routes passed write_sentinel=False and relied on the wizard UI explicitly calling POST /api/install/complete). A UI client that never calls /complete, or an answer-file/API-up hal0 setup install, previously left first_run depending only on the has_models heuristic. Now any successful apply — CLI in-process, /apply, or /apply-selections — writes the sentinel itself.
• mark_first_run_done() is idempotent, so the wizard's existing POST /complete call after apply remains a harmless no-op — no behavior change there.
• Response shapes are unchanged: /apply still returns {tier, model_ids, slots, next}, /apply-selections still returns {model_ids, slots}.
• Did not touch setup_command.py, answers.py, setup_plan.py, setup_install.py, or add any /api/config/urls route (owned by sibling PR installer-setup WS-C: network coherence backend (unify HAL0_BIND_HOST + seed origins/hostname) #1099).

Test plan

• uv run ruff format src tests — 1 file reformatted (test file), rest unchanged
• uv run ruff check src tests — all checks passed
• uv run ruff format --check src tests — all files already formatted
• uv run pytest tests/api/test_installer_routes.py tests/api/test_install_apply.py tests/api/test_apply_selections.py tests/install/test_orchestrate.py -q — 34 passed
• uv run pytest tests/ -q --ignore=tests/e2e — 4617 passed, 14 failed, 1 error, all pre-existing/unrelated (hermes venv/wrapper, comfyui phase4, proxmox host detection, container spec dispatch, config-url-proxy, models-preview, settings-store, openwebui prewire smoke)
• New tests added:
  • tests/api/test_install_apply.py::test_apply_writes_first_run_sentinel — /apply writes the sentinel and flips first_run without any POST /complete.
  • tests/api/test_install_apply.py::test_apply_delegates_to_apply_selections_core — asserts /apply calls the shared _apply_selections_core (spies on it).
  • tests/api/test_apply_selections.py::test_apply_selections_writes_first_run_sentinel — /apply-selections writes the sentinel and flips first_run.

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