Implement OOBGC and Async Fiber domain sweeps with OOM protection

[ayush6288]

Background

This PR introduces:

1. Out-of-Band GC middleware that defers GC to after the response, with mid-flight OOM protection via RSS monitoring.
2. A Sequel extension for memory guard on DB queries.
3. Async garbage collector bin using Ruby 3 Fibers and non-blocking HTTP for blobstore cleanup.

Design

See the PR description for full architectural details. All changes are opt-in via feature flags and configuration.