Simplify sync metrics to track only newly saved events

Replace broken event counting that occurred before duplicate/policy checks
with accurate tracking of events that are new, accepted, and saved.

Changes:
- Added ProcessResult enum to track event processing outcomes
- Modified process_event_static() to return ProcessResult
- Replaced events_total (with source labels) with events_synced_total
- Removed gap_events_total and event_source module
- Removed eose_received flag (EOSE is per-subscription, not suitable)
- Updated all tests to use new simplified API

The new ngit_sync_events_synced_total metric only counts events that:
1. Are new (not duplicates)
2. Pass write policy validation
3. Are successfully saved to database

All 165 tests pass (124 lib + 41 integration)

1 files changed, 3 insertions, 3 deletions

diff --git a/tests/common/sync_helpers.rs b/tests/common/sync_helpers.rs
index 8971369..cccfbdd 100644
--- a/tests/common/sync_helpers.rs
+++ b/tests/common/sync_helpers.rs
@@ -698,9 +698,9 @@ impl ParsedMetrics {
 
     // Convenience accessors for sync metrics
 
-    /// Get total events synced from a specific source
-    pub fn events_total(&self, source: &str) -> Option<u64> {
-        self.counter("ngit_sync_events_total", &[("source", source)])
+    /// Get total events synced (no source categorization)
+    pub fn events_synced_total(&self) -> Option<u64> {
+        self.counter("ngit_sync_events_synced_total", &[])
     }
 
     /// Check if a specific relay is connected