feat(events): accept 'TBD' as a valid location

Events are often scheduled before the platform (Zoom link, Slack
channel) is chosen. The current workaround is a placeholder URL like
"https://us02web.zoom.us/j/TBD", which leaks to the public page as a
broken link.

Accept the literal "TBD" (case-insensitive) in both the Mongoose
validator and the form-side validator. The public detail page renders
"Platform TBD" instead of a link when the location matches.

app/pages/events/[slug].vue

@@ -22,7 +22,10 @@
         </div>
         <div class="event-meta-item">
           <span class="meta-label">Location</span>
-          {{ event.location }}
+          <span v-if="event.location?.trim().toUpperCase() === 'TBD'">
+            Platform TBD
+          </span>
+          <template v-else>{{ event.location }}</template>
         </div>
         <div v-if="event.circle" class="event-meta-item">
           <CircleBadge :circle="event.circle" />