fix: rename existing system categories to PascalCase-with-dashes via migration

Migration 0007 converts all scope='system' category names in-place
(e.g. "invoices" → "Invoices", "vendor-invoices" → "Vendor-Invoices").

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

features/doc-service/CLAUDE.md

@@ -117,6 +117,7 @@ Unique constraint: `(document_id, group_id)`
 | `0004` | `add_document_shares` |
 | `0005` | `add_share_can_delete` |
 | `0006` | `add_category_scope` |
+| `0007` | `capitalize_system_category_names` |
 
 ---