feat: implement efficient upsert operation for create_or_update

Replace the existing two-query create_or_update implementation with a
single atomic PostgreSQL upsert using ON CONFLICT clause to eliminate
race conditions and improve performance.

Race condition fix:
The previous implementation had a critical race condition where
multiple concurrent requests could:
1. Both call find() and get None (record doesn't exist)
2. Both call create() and the second one fails with duplicate key
   error
3. Or between find() and create(), another transaction inserts the
   record

This created unreliable behavior in high-concurrency scenarios.

Changes:
- Add generate_upsert_query function in trait_implementation.rs
- Generate SQL with INSERT ... ON CONFLICT ... DO UPDATE SET pattern
- Remove default trait implementation that used separate
  find/create/update calls
- Update derive_trait to include upsert query generation
- Convert create_or_update from default implementation to required
  trait method

The new implementation eliminates race conditions while reducing
database round trips from 2-3 queries down to 1, significantly
improving both reliability and performance.

README.md

@@ -506,7 +506,6 @@ Georm is designed for zero runtime overhead:
 
 ### High Priority
 - **Transaction Support**: Comprehensive transaction handling with atomic operations
-- **Race Condition Fix**: Database-native UPSERT operations to replace current `create_or_update`
 
 ### Medium Priority
 - **Multi-Database Support**: MySQL and SQLite support with feature flags