feat: add outbox

Signed-off-by: kjuulh <contact@kjuulh.io>

crates/crunch-postgres/build.rs

@@ -0,0 +1,5 @@
+// generated by `sqlx migrate build-script`
+fn main() {
+    // trigger recompilation when a new migration is added
+    println!("cargo:rerun-if-changed=migrations");
+}