feat(tenant): add description template field to tenant settings

[jeffreyvdhondel]

What & why

Adds an optional, per-tenant Idea Template that prefills the description editor when a user opens the "Share your idea" modal. Admins configure a structured template (e.g. What is your idea? / What problem does
it solve? / Who benefits?) so submissions arrive with consistent, useful detail instead of a blank box.

The setting follows the same pattern as the existing WelcomeMessage / Invitation tenant fields, so it slots into the current architecture with no new concepts.

Behaviour

• Admin → General Settings gains an "Idea Template" textarea (Markdown supported).
• When a template is configured, the new-post description editor opens prefilled with it, and title auto-population is suppressed so the template heading doesn't leak into the title.
• No template configured → empty editor (unchanged default experience).
• An in-progress cached draft always takes precedence over the template, so drafts are never overwritten.

Changes

Backend :

• Migration 202606081200_add_tenant_description_template.sql adds tenants.description_template TEXT NOT NULL DEFAULT ''
• description_template threaded through the tenant entity, DB scan struct + ToModel, the two tenant SELECTs, the settings UPDATE, the UpdateTenantSettings command/action (incl. a 2000-char validation), and
  the admin handler

Frontend:

• descriptionTemplate added to the Tenant model and the update-settings request payload
• New "Idea Template" field on the General Settings admin page
• ShareFeedback.tsx prefills the editor from the tenant template when no draft exists

Notes

• Backwards compatible: column defaults to '', so existing tenants behave exactly as before until they opt in.