feat(sync): implement pagination for historic_sync REQ+EOSE flow

Add automatic pagination support for non-Negentropy historic sync to handle
large result sets efficiently. When a subscription receives >= 75 events,
the system automatically fetches the next page using the 'until' parameter.

Changes:
- Add PaginationState struct to track event counts and min timestamps
- Add pagination_state HashMap to PendingBatch for per-subscription tracking
- Add PAGINATION_THRESHOLD constant (75 events)
- Pass pending_sync_index to event processor for state updates
- Track events and timestamps as they arrive
- Check threshold on EOSE and launch follow-up subscriptions
- Initialize pagination state when creating historic sync subscriptions
- Update test fixtures in algorithms.rs

The pagination continues recursively until a page returns fewer than 75 events,
ensuring complete historic data retrieval without overwhelming relay limits.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/sync/relay_connection.rs b/src/sync/relay_connection.rs
index 37094be..5a61777 100644
--- a/src/sync/relay_connection.rs
+++ b/src/sync/relay_connection.rs
@@ -23,8 +23,8 @@ use crate::nostr::builder::SharedDatabase;
 /// Events from a relay connection
 #[derive(Debug)]
 pub enum RelayEvent {
-    /// A new event was received
-    Event(Event),
+    /// A new event was received (event, subscription_id)
+    Event(Event, SubscriptionId),
     /// End of stored events for a subscription
     EndOfStoredEvents(SubscriptionId),
     /// Connection was closed
@@ -216,7 +216,7 @@ impl RelayConnection {
                                 sub_id = %subscription_id,
                                 "Received event"
                             );
-                            if event_sender.send(RelayEvent::Event(*event)).await.is_err() {
+                            if event_sender.send(RelayEvent::Event(*event, subscription_id.clone())).await.is_err() {
                                 tracing::debug!(relay = %url, "Event sender closed, stopping event loop");
                                 break;
                             }