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/read_model.go

@@ -1,19 +1,23 @@
 package eventstore
 
-import "time"
+import (
+	"time"
+
+	"github.com/shopspring/decimal"
+)
 
 // ReadModel is the minimum representation of a read model.
 // It implements a basic reducer
 // it might be saved in a database or in memory
 type ReadModel struct {
-	AggregateID       string    `json:"-"`
-	ProcessedSequence uint64    `json:"-"`
-	CreationDate      time.Time `json:"-"`
-	ChangeDate        time.Time `json:"-"`
-	Events            []Event   `json:"-"`
-	ResourceOwner     string    `json:"-"`
-	InstanceID        string    `json:"-"`
-	Position          float64   `json:"-"`
+	AggregateID       string          `json:"-"`
+	ProcessedSequence uint64          `json:"-"`
+	CreationDate      time.Time       `json:"-"`
+	ChangeDate        time.Time       `json:"-"`
+	Events            []Event         `json:"-"`
+	ResourceOwner     string          `json:"-"`
+	InstanceID        string          `json:"-"`
+	Position          decimal.Decimal `json:"-"`
 }
 
 // AppendEvents adds all the events to the read model.