add tags

Trip-level tags: admins configure available tags in the trip header (inline add/remove pills). The AI can also create new tags via the add_tag tool.
Event tags: selectable in the Add Event modal, shown as colored pills on event cards in the timeline, and on AI suggestion cards.
AI integration: sees available tags in its system prompt, applies them when proposing events, can create new ones with add_tag.
Discord: tags shown as inline code blocks under each event in /trip view.
Colors: auto-assigned from an 8-color palette by tag index, consistent everywhere.

drb-frontend/lib/types.ts

@@ -110,6 +110,7 @@ export interface TripEvent {
   maps_link: string | null;
   place_id: string | null;
   notes: string | null;
+  tags: string[];
   attendees: Record<string, string>;
   created_at: string;
 }
@@ -132,6 +133,7 @@ export interface TripRecord {
   start_date: string;
   end_date: string;
   attendees: Record<string, string>;
+  available_tags: string[];
   created_at: string;
   events?: TripEvent[];
 }