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/c2api.ts

@@ -142,6 +142,8 @@ export const c2api = {
     request<import("@/lib/types").TripRecord>("/trips", { method: "POST", body: JSON.stringify(body) }),
   deleteTrip: (id: string) =>
     request(`/trips/${id}`, { method: "DELETE" }),
+  updateTripTags: (id: string, available_tags: string[]) =>
+    request<{ available_tags: string[] }>(`/trips/${id}/tags`, { method: "PUT", body: JSON.stringify({ available_tags }) }),
   createTripEvent: (tripId: string, body: object) =>
     request<import("@/lib/types").TripEvent>(`/trips/${tripId}/events`, { method: "POST", body: JSON.stringify(body) }),
   deleteTripEvent: (tripId: string, eventId: string) =>