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/evaluator/src/eval_loop.rs

@@ -528,6 +528,7 @@ async fn check_declarative_config(
         branch: None,
         scheduling_shares: None,
         state: None,
+        keep_nr: None,
       };
       if let Err(e) = repo::jobsets::upsert(pool, input).await {
         tracing::warn!("Failed to upsert declarative jobset: {e}");