refactor(data): drop legacy toilet boolean columns from pois

## What to build Drop the legacy toilet boolean columns from the `pois` table once the category-based model is bedded down and nothing in NavigaTUM still reads them as a pre-`category` proxy. There are no external consumers, so the gate is purely self-use. Note: the toilet attributes (gender, wheelchair, shower) remain semantically valuable as toilet-specific attribute columns, separate from the `category` pivot. This slice is two pieces of work: 1. Audit the codebase for remaining reads of the toilet booleans where `category` should be used instead. Migrate any such reads. 2. Decide column-by-column whether to drop or retain. Drop the ones that exist purely as a pre-`category` categorisation proxy. Retain (with an ADR or migration comment) the ones that are still earning their keep as typed attributes. If after audit no columns are dropped, this slice produces only the ADR documenting the model — and that is still a useful outcome. ## Acceptance criteria - [ ] Codebase audit confirms no remaining reads of toilet booleans as a pre-`category` proxy. - [ ] Migration drops or retains each boolean column based on the audit outcome. - [ ] Where columns are retained, an ADR or migration comment documents why. - [ ] Where columns are dropped, dependent code is migrated and tests pass. - [ ] `pois_tiles` MVT contract is updated if columns are dropped.