feat: fix time slot hierarchy — seeder, API include_children, frontend dropdown, navigation

Restructure the festival hierarchy end-to-end:

Seeder: Remove duplicate festival-level VOLUNTEER time slots, keep only CREW
operational slots. Rename sub-events to "Dag 1/2/3 — ..." pattern. Change
Nachtsecurity to Security (cross_event). EHBO/Security shifts now use sub-event
time slots via cross_event exception. Add flat event "Braderie Dorpstown 2026".

API: Add ?include_children=true to TimeSlotController for festivals, returning
all sub-event time slots with source and event_name fields. Update
StoreShiftRequest and UpdateShiftRequest to accept child time slots for
cross_event sections.

Frontend: Create useTimeSlotDropdown composable with 4-scenario dropdown logic.
Replace AppSelect with VAutocomplete in CreateShiftDialog with grouped items,
dimmed festival slots, and info tooltips. Add InfoTooltip reusable component.
Show festival context labels on cross_event sections in sub-event section lists.
Add read-only festival time slots on sub-event time-slots page. Add cross_event
context banner with "Bekijk alle diensten" link.

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

apps/app/src/components/common/InfoTooltip.vue

@@ -0,0 +1,20 @@
+<template>
+  <VTooltip
+    location="bottom"
+    max-width="320"
+    open-on-click
+  >
+    <template #activator="{ props: tooltipProps }">
+      <VIcon
+        v-bind="tooltipProps"
+        icon="tabler-info-circle"
+        size="16"
+        color="medium-emphasis"
+        class="cursor-help"
+      />
+    </template>
+    <div class="text-body-2">
+      <slot />
+    </div>
+  </VTooltip>
+</template>