Merge relay.ngit.dev migration: bug fixes and migration tooling

This merge includes critical bug fixes and comprehensive migration tooling
developed during the relay.ngit.dev migration effort.

Bug Fixes:
- Fix git protocol error handling to return HTTP 200 with ERR pkt-line
- Fix naughty list false positives and DNS failure identification
- Fix database query filters in load_existing_events (remove .since())
- Fix OID fetch tracking to distinguish 0 OIDs from successful fetches
- Fix purgatory event source tracking for filtered expiry logging
- Implement OID retry logic for 'not our ref' errors

Migration Tools & Documentation:
- Complete 5-phase migration analysis pipeline with orchestration script
- Phase 1: Event fetching from source relay
- Phase 2: Git sync verification
- Phase 3: Categorization and relay comparison
- Phase 4: Log extraction (parse failures, purgatory expiry)
- Phase 5: Action classification for migration decisions
- Comprehensive migration guide with lessons learned
- Troubleshooting guide for permission and corruption issues

Configuration:
- Add NGIT_LOG_LEVEL configuration option
- Update git throttle limits to 60/minute
- Improve logging throughout for better observability

1 files changed, 15 insertions, 7 deletions

diff --git a/src/purgatory/sync/functions.rs b/src/purgatory/sync/functions.rs
index 65d29af..9207d58 100644
--- a/src/purgatory/sync/functions.rs
+++ b/src/purgatory/sync/functions.rs
@@ -368,15 +368,23 @@ pub async fn sync_identifier_from_url<C: SyncContext + ?Sized>(
     let fetch_result = ctx.fetch_oids(&target_repo, url, &needed_oids).await;
     throttle_manager.complete_request(&domain);
 
-    let oids_fetched = match fetch_result {
-        Ok(fetched) => {
+    let fetched_oids = match fetch_result {
+        Ok(fetched) if !fetched.is_empty() => {
             debug!(
                 identifier = %identifier,
                 url = %url,
                 oids_fetched = fetched.len(),
                 "Fetch succeeded"
             );
-            fetched.len()
+            fetched
+        }
+        Ok(_) => {
+            debug!(
+                identifier = %identifier,
+                url = %url,
+                "Fetch returned no OIDs (not available on remote)"
+            );
+            vec![]
         }
         Err(e) => {
             debug!(
@@ -385,13 +393,13 @@ pub async fn sync_identifier_from_url<C: SyncContext + ?Sized>(
                 error = %e,
                 "Fetch failed"
             );
-            0
+            vec![]
         }
     };
 
     // Try to process any events that can now be satisfied
-    if oids_fetched > 0 {
-        let new_oids: HashSet<String> = needed_oids.into_iter().collect();
+    if !fetched_oids.is_empty() {
+        let new_oids: HashSet<String> = fetched_oids.iter().cloned().collect();
         if let Err(e) = ctx
             .process_newly_available_git_data(&target_repo, &new_oids)
             .await
@@ -404,7 +412,7 @@ pub async fn sync_identifier_from_url<C: SyncContext + ?Sized>(
         }
     }
 
-    oids_fetched
+    fetched_oids.len()
 }
 
 /// Sync git data for an identifier.