feat: generate transcript and event-driven particle processing

This generates the transcript and shows the caption experience on the
client side for media particles. It also simplifies other side effects
that we must perform such as updating the `last_child_created_at` field
for stream and container particles.

js/src/hooks/use-create-particle.ts

@@ -1,5 +1,5 @@
 import { useMutation } from "@tanstack/react-query";
-import { createParticle, updateStreamLastChildAt } from "@/lib/firestore-particles";
+import { createParticle } from "@/lib/firestore-particles";
 import type { ParticleType, ParticlePropertiesMap } from "@/api/types";
 import { particlePath, ParticlePath, toFirestoreChildrenPath, toFirestoreDocPath } from "@/lib/particle-path";
 
@@ -15,16 +15,12 @@ export function useCreateParticle() {
   return useMutation({
     mutationFn: async (params: CreateParticleParams) => {
       const collectionPath = toFirestoreChildrenPath(params.path);
-      const result = await createParticle(
+      return await createParticle(
         collectionPath,
         params.type,
         params.properties,
         params.createdByHumanId,
       );
-
-      const streamDocPath = toFirestoreDocPath(params.path);
-      await updateStreamLastChildAt(streamDocPath);
-      return result;
     }
   });
 }