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)

0 files changed, 0 insertions, 0 deletions