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, 8 insertions, 0 deletions

diff --git a/src/nostr/policy/pr_event.rs b/src/nostr/policy/pr_event.rs
index 00e09c3..072e445 100644
--- a/src/nostr/policy/pr_event.rs
+++ b/src/nostr/policy/pr_event.rs
@@ -127,6 +127,10 @@ impl PrEventPolicy {
                 .ok_or_else(|| anyhow::anyhow!("No identifier in PR event"))?;
 
             // Fetch repository data
+            // NOTE: Only fetch from database, NOT purgatory. Incoming PR events should
+            // only be accepted for announcements that have been promoted (validated).
+            // If the announcement is still in purgatory, the PR event should also go
+            // to purgatory and wait for the announcement to be promoted.
             let db_repo_data = fetch_repository_data(&self.ctx.database, &identifier).await?;
 
             // Extract owner pubkey from source repo path
@@ -203,6 +207,10 @@ impl PrEventPolicy {
         let identifier = parts[2];
 
         // 2. Fetch repo data
+        // NOTE: Only fetch from database, NOT purgatory. Incoming PR events should
+        // only be accepted for announcements that have been promoted (validated).
+        // If the announcement is still in purgatory, the PR event should also go
+        // to purgatory and wait for the announcement to be promoted.
         let db_repo_data = fetch_repository_data(&self.ctx.database, identifier).await?;
 
         // 3. Extract list of maintainers from "a 30617:<maintainer>:<identifier>" tags