fc-common: add GC pinning and machine health infrastructure

Migration 017 adds `builds.keep`, `jobsets.keep_nr`, and health tracking
columns to `remote_builders`. Repo layer implements `set_keep`,
`list_pinned_ids`, `record_failure` with exponential backoff,
`record_success`, and `find_for_system` filtering of disabled builders.
GC root cleanup now skips pinned builds.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ibba121de3dc42f71204e3a8f5776aa8b6a6a6964

crates/server/tests/e2e_test.rs

@@ -64,6 +64,7 @@ async fn test_e2e_project_eval_build_flow() {
     branch:            None,
     scheduling_shares: None,
     state:             None,
+    keep_nr:           None,
   })
   .await
   .expect("create jobset");