Add description and assumptions fields (rich text) to feature templates

[NickMonrad]

Overview

Feature templates in the template library currently only support task-level data (name, hours, resource type). They should also support description and assumptions fields using the TipTap rich text editor, consistent with how these fields work on epics, features, and stories in the backlog.

Scope

Template-level fields

• Add description (rich text) to FeatureTemplate — appears in the template editor and is applied when the template is used
• Add assumptions (rich text) to FeatureTemplate — same

Template task-level fields

• Add description (rich text) to TemplateTask — propagated to the generated Task when the template is applied to a feature

Behaviour when applying a template

When a FeatureTemplate is applied to a Feature:

• The feature's description should be populated from the template's description (if the feature's description is currently empty)
• The feature's assumptions should be populated from the template's assumptions (same condition)
• Each generated Task should carry the corresponding TemplateTask.description

UI

• Template editor (create/edit modal or inline editor) should show TipTap rich text fields for description and assumptions
• Use the existing RichTextEditor component from client/src/components/shared/RichTextEditor.tsx

Schema changes required

• FeatureTemplate: add description String? and assumptions String?
• TemplateTask: add description String?
• Migration needed