refactor(structure): group platform, infra, and shared packages

Reorganize internal packages into platform/infra/shared layers and update
imports, docs, and seed-count tests to match current system configs.

internal/infra/config/config_test.go

@@ -0,0 +1,14 @@
+package config
+
+import "testing"
+
+func TestApplyEnvOverridesRedisMaintNotifications(t *testing.T) {
+	t.Setenv("REDIS_MAINT_NOTIFICATIONS", "true")
+
+	cfg := &configModel{}
+	applyEnvOverrides(cfg)
+
+	if !cfg.Redis.MaintNotifications {
+		t.Fatal("REDIS_MAINT_NOTIFICATIONS=true was not applied")
+	}
+}