feat(db & e2e): Enhance DB Schemas/Controllers and Improve E2E Tests (#966)

* feat: add global teardown to remove test data and add registration/log-out to auth flow

* refactor(models/Conversation): index user field and add JSDoc to deleteConvos

* refactor: add user index to message schema and ensure user is saved to each Message

* refactor: add user to each saveMessage call

* fix: handle case where title is null in zod schema

* feat(e2e): ensure messages are deleted on cleanUp

* fix: set last convo for all endpoints on conversation update

* fix: enable registration for CI env

api/models/schema/convoSchema.js

@@ -17,6 +17,7 @@ const convoSchema = mongoose.Schema(
     },
     user: {
       type: String,
+      index: true,
       default: null,
     },
     messages: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Message' }],