feat: add embedded testing server for postgres (#9955)

# Which Problems Are Solved

1. there was no embedded database to run tests against
2. there were no tests for postgres/migrate
3. there was no test setup for repository which starts a client for the
embedded database

# How the Problems Are Solved

1. postgres/embedded package was added
2. tests were added
3. TestMain was added incl. an example test

# Additional Changes

none

# Additional Context

closes #9934

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/v3/storage/database/database.go

@@ -8,6 +8,7 @@ import (
 type Pool interface {
 	Beginner
 	QueryExecutor
+	Migrator
 
 	Acquire(ctx context.Context) (Client, error)
 	Close(ctx context.Context) error
@@ -17,6 +18,7 @@ type Pool interface {
 type Client interface {
 	Beginner
 	QueryExecutor
+	Migrator
 
 	Release(ctx context.Context) error
 }