chore: set application to validate mode and add database initialization script

infrastructure/src/main/resources/db-scripts/V1.0.0_DDL_init_scheme.sql

@@ -0,0 +1,8 @@
+CREATE TABLE restemailbridge.mail
+(
+    id       BIGSERIAL PRIMARY KEY,
+    body     VARCHAR(255) NOT NULL,
+    recipient VARCHAR(255) NOT NULL,
+    sender   VARCHAR(255) NOT NULL,
+    subject  VARCHAR(255) NOT NULL
+);