V117: Document composition — template category, document sections, template presets.

Three schema changes for the document composition feature:

1. ALTER phoenix_kit_doc_templates — adds a nullable category :: varchar column and an index on (category) for category-filtered queries.

2. CREATE phoenix_kit_doc_document_sections — join table between documents and templates for multi-section composed documents. Each row represents one template-backed section at a specific position within a document. Supports per-section variable overrides (variable_values) and image configuration (image_params). Positions are unique per document; deleting a document cascades to its sections. Nullifying a template FK on delete allows the section to survive template removal (content would need regeneration).

3. CREATE phoenix_kit_doc_template_presets — named, reusable compositions of template sections. Presets are scoped via scope_type + scope_id (e.g. "organization" + org uuid) and optionally categorized. The sections JSONB column stores an ordered array of section descriptors ([%{template_uuid, position, variable_values, image_params}]).

All operations use IF NOT EXISTS guards so re-running up/1 on a partially-applied schema is a no-op.

down/1 is destructive for the category column: dropping it deletes any values users have assigned. The two new tables drop cleanly (re-running up/1 recreates them empty), but column data is lost on rollback by design — Ecto.Migration's rollback contract has no way to preserve it.