Metrics and logs - MongoDB Submission (#43)

Adds MongoDB-backed submission for survey statistics/metrics and introduces MongoDB ObjectID-based participant/task identifiers to support upserts and linking metrics to a participant.

It introduces breaking changes to the exsisting json stat and metric files. Delete the old ones

Changes:

Add internal/storage/MongoStorage and wire pm survey submit / pm start to submit local .pm/*.json files to MongoDB.
Extend stats/metrics models to include MongoDB _id / participant_id fields and add participant ID propagation into task metrics.
Update app initialization to generate ObjectIDs for new participants and expose GetParticipantID() via StatsService.

internal/commands/survey/root.go

@@ -26,11 +26,6 @@ Your responses will be kept confidential and used solely for research purposes.`
 var RootCmd = &cobra.Command{
 	Use:  "survey",
 	Long: long,
-	PersistentPreRun: func(cmd *cobra.Command, args []string) {
-		if app != nil {
-			cmd.SetContext(context.WithValue(cmd.Context(), appKey, app))
-		}
-	},
 }
 
 func SetApp(application *App) {