fix: prune stale concept aliases during ingest

[yibeichan]

Summary

• prune concept_aliases rows whose duplicate or canonical slugs no longer exist after a fresh ingest
• keep valid alias rows across rebuilds
• add regressions for stale alias pruning and valid alias preservation

Closes #213.

Tests

• uv run --with pytest --with ./cli python -m pytest cli/tests/test_ingest.py::test_ingest_prunes_concept_aliases_for_missing_concepts -q
• uv run --with pytest --with ./cli python -m pytest cli/tests/test_sync.py::TestRebuildIndexSideTablePreservation::test_concept_aliases_survive_rebuild -q
• uv run --with pytest --with ./cli python -m pytest cli/tests/test_ingest.py cli/tests/test_sync.py -q
• cd cli && uv run --with pytest --with . python -m pytest tests/