V120: Document Creator Category → Type taxonomy.

Creates two tables — phoenix_kit_doc_categories and phoenix_kit_doc_types — and adds nullable category_uuid / type_uuid FK columns to phoenix_kit_doc_templates and phoenix_kit_doc_documents.

Data migration: each distinct non-empty legacy category string on templates becomes a Category row (matched case-insensitively, so "Financial" and "financial" collapse into one); templates are repointed via category_uuid; documents inherit their template's category. The legacy category string columns on templates and presets are then dropped. type_uuid stays NULL everywhere (no Types yet).

Note: phoenix_kit_doc_template_presets does not get a category_uuid column — presets do not join the new taxonomy, and their legacy category strings are discarded (not migrated). Dropping the preset category column also drops the V117 phoenix_kit_doc_template_presets_scope_index, which this migration recreates on (scope_type, scope_id).

All operations are idempotent.