fc-queue-runner: implement persistent notification retry queue with exponential backoff

Adds a `notification_tasks` table and a background worker to (hopefully
reliably) deliver webhooks, git status updates, and e-mail notifications
with automatic retry on transient failures.

This was one of the critical gaps, finally done.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I794967c66958658c4d8aed40793d67f96a6a6964

crates/common/src/repo/mod.rs

@@ -10,6 +10,7 @@ pub mod failed_paths_cache;
 pub mod jobset_inputs;
 pub mod jobsets;
 pub mod notification_configs;
+pub mod notification_tasks;
 pub mod project_members;
 pub mod projects;
 pub mod remote_builders;