feat: extract into remaining packages

There has been a cyclic dependency.
transaction
	-> treasure_chest
	-> transaction_recurring
	-> transaction

This has been temporarily solved by moving the GenerateTransactions
function into the transaction package. In the future, this function has
to be rewritten to use a proper Service insteas of direct DB access or
replaced with a different system entirely.

internal/handler/middleware/security_headers.go

@@ -2,10 +2,10 @@ package middleware
 
 import (
 	"net/http"
-	"spend-sparrow/internal/types"
+	"spend-sparrow/internal/core"
 )
 
-func SecurityHeaders(serverSettings *types.Settings) func(http.Handler) http.Handler {
+func SecurityHeaders(serverSettings *core.Settings) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			w.Header().Set("X-Content-Type-Options", "nosniff")