[thorwhalen]

Closes #112.

Adds nw.adopt_tenant_id(project, *, dry_run=False): an explicit, idempotent one-shot that writes a legacy project's currently-resolved tenant (root+title hash, or song hash) into project.json as tenant_id. The tenant, and so the data the project reads, is unchanged; the key just stops depending on root + title, so a folder removed and recreated by hand can no longer inherit the old Postgres graph. Projects that already have a tenant_id are left alone; nw never calls it itself (the host or a migration command runs it, ideally while the project is intact, since the song-hash form reads the audio file). It adds one key to project.json; no graph data is moved or deleted. Running it over a real deployment's existing projects is the host's call.

Tests: tests/test_tenant_id.py (pins the same tenant, idempotent, dry-run writes nothing, key survives retitle/move, non-project returns None). Full suite 761 passed.

Independent review (fresh Opus): two confirmed problems, both fixed here: (1) a missing song file would have pinned the root+title fallback permanently, so adoption now raises FileNotFoundError instead; (2) a self-keyed storage (MappingStorage) has no tenant_id, so adoption now returns None for it. Noted, not changed: two legacy projects that share audio bytes already share one tenant and adoption keeps that. No path moves or deletes graph data.