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/models/app.go

@@ -3,6 +3,8 @@ package models
 import (
 	"context"
 	"log/slog"
+
+	"go.mongodb.org/mongo-driver/bson/primitive"
 )
 
 type App struct {
@@ -57,5 +59,6 @@ type StatsService interface {
 	Load(ctx context.Context) error
 	Save(ctx context.Context) error
 	GetStatistics() (Statistics, error)
+	GetParticipantID() primitive.ObjectID
 	RecordTaskRun(ctx context.Context, run TaskRunMetrics) error
 }