fix(nostrrelay): populate size field for event storage accounting

Changes:
- relay/event.py: Add `size: int = 0` field to NostrEvent model
- relay/client_connection.py: Set `event.size = event.size_bytes` when creating events from WebSocket messages

The size field has existed in the database schema since migration m001 but was never populated, causing:
  - Incorrect storage accounting (always 0)
  - Broken storage quota enforcement
  - Failed event pruning when storage limits reached

The size field is internal relay metadata and is excluded from the nostr_dict() output, maintaining NIP-01 compliance. The size_bytes property calculates the actual byte size of the event's JSON representation.

Fixes: Database constraint violation when inserting events without the required size column value.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

relay/client_connection.py

@@ -121,6 +121,8 @@ class NostrClientConnection:
             }
 
             event = NostrEvent(**event_dict)
+            # Set the size field from the size_bytes property
+            event.size = event.size_bytes
             await self._handle_event(event)
             return []
         if message_type == NostrEventType.REQ: