Use partial indexes for nullable relationships

[jechol]

Summary

• Nullable belongs_to relationships now get partial indexes excluding NULL rows: references use index_where: :not_nil, custom indexes use where: "fk_id IS NOT NULL". FK lookups are equality matches (always IS NOT NULL), so the smaller index serves them fully. allow_nil? false relationships keep full indexes.
• On multitenant resources, the tenant attribute gets its own single-column index when every other FK index is partial, since partial FK indexes cannot serve tenant-only lookups.
• Combined with fix: generate full foreign key indexes #2 on the multitenant single-column index: all_tenants?: true + include_base_filter?: false + stable {table}_{fk_id}_fkey_index name, plus the partial where when nullable.
• Bumps version to 0.5.0 with changelog and README updates.

Requires ash_postgres with index_where on references (ash-project/ash_postgres#795) — not yet in a hex release, dependency stays on GitHub main.

Test plan

• mix test (22 tests, including new test/partial_index_test.exs)