feat: announcement purgatory

Extends purgatory to hold repository announcements until git data arrives,
preventing empty repositories from being served to clients.

When an announcement is received, a bare repo is created immediately and the
announcement is held in purgatory. It is only promoted and served once a git
push confirms real content exists. If no push arrives before expiry, the bare
repo is deleted and the announcement is silently discarded.

Key behaviours:
- Soft expiry: announcements are hidden from clients but kept alive while git
  pushes are in progress, reviving on successful push
- Expiry is extended when a matching state event or git push is observed
- NIP-09 deletion events remove announcements from purgatory
- Purgatory state (announcements, state events, PR events, expired set) is
  persisted to disk on graceful shutdown and restored on startup, with elapsed
  downtime subtracted from expiry deadlines
- Purgatory announcements drive StateOnly sync in the sync system so state
  events are fetched from listed relays before promotion
- SyncLevel added to RepoSyncIndex to distinguish purgatory repos (StateOnly)
  from promoted repos (Full L2+L3 sync)

1 files changed, 5 insertions, 0 deletions

diff --git a/src/nostr/policy/related.rs b/src/nostr/policy/related.rs
index 7ce87db..cfe04a7 100644
--- a/src/nostr/policy/related.rs
+++ b/src/nostr/policy/related.rs
@@ -139,6 +139,11 @@ impl RelatedEventPolicy {
                 .push((addr, pubkey, identifier));
         }
 
+        // NOTE: Intentionally only checks the database (promoted announcements), not purgatory.
+        // Related events should only be accepted once the repository announcement has been
+        // validated (promoted via git data). Events referencing purgatory-only repositories
+        // are correctly rejected as orphans and can be re-submitted after promotion.
+
         // Query each kind group
         for (kind, refs) in by_kind {
             let authors: Vec<PublicKey> = refs.iter().map(|(_, pk, _)| *pk).collect();