fix(eventstore): precise decimal (#8527)

# Which Problems Are Solved

Float64 which was used for the event.Position field is [not precise in
go and gets rounded](https://github.com/golang/go/issues/47300). This
can lead to unprecies position tracking of events and therefore
projections especially on cockcoachdb as the position used there is a
big number.

example of a unprecies position:
exact: 1725257931223002628
float64: 1725257931223002624.000000

# How the Problems Are Solved

The float64 was replaced by
[github.com/jackc/pgx-shopspring-decimal](https://github.com/jackc/pgx-shopspring-decimal).

# Additional Changes

Correct behaviour of makefile for load tests.
Rename `latestSequence`-queries to `latestPosition`

internal/eventstore/repository/mock/repository.mock.impl.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/shopspring/decimal"
 	"github.com/stretchr/testify/assert"
 	"go.uber.org/mock/gomock"
 
@@ -186,8 +187,8 @@ func (e *mockEvent) Sequence() uint64 {
 	return e.sequence
 }
 
-func (e *mockEvent) Position() float64 {
-	return 0
+func (e *mockEvent) Position() decimal.Decimal {
+	return decimal.Decimal{}
 }
 
 func (e *mockEvent) CreatedAt() time.Time {