feat: registration section preferences with show_in_registration filtering and deduplication

Add show_in_registration and registration_description columns to festival_sections.
Registration form now shows deduplicated sections by name (across sub-events),
filtered by show_in_registration=true, grouped by category with card-based UI.
Section preferences use section_name instead of section_id.
Add GET/PUT registration-settings endpoints for festival-level bulk management.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dev-docs/SCHEMA.md

@@ -295,6 +295,8 @@ scopeFestivals()    // WHERE event_type IN ('festival', 'series')
 | `crew_accreditation_level`        | string nullable    | **v1.5** Default accreditation level for crew (e.g. AAA, AA, A)   |
 | `public_form_accreditation_level` | string nullable    | **v1.5** Accreditation level for public form registrants          |
 | `timed_accreditations`            | bool               | **v1.5** Accreditations are time-limited for this section         |
+| `show_in_registration`            | bool               | **v1.8** Show this section in the volunteer registration form     |
+| `registration_description`        | text nullable      | **v1.8** Description shown to volunteers in the registration form |
 | `deleted_at`                      | timestamp nullable | Soft delete                                                       |
 
 **Relations:** `hasMany` shifts  
@@ -309,6 +311,7 @@ scopeFestivals()    // WHERE event_type IN ('festival', 'series')
 - `added_to_timeline`: false
 - `responder_self_checkin`: true
 - `timed_accreditations`: false
+- `show_in_registration`: false
 
 > **Note:** "Overkoepelende" sections (shared across all sub-events of a festival)
 > are identified by `type = 'cross_event'`. There is no separate `is_shared` boolean