feat: Add high-level integration testing framework

- Created trait abstractions for dependency injection with mockall support
- Added comprehensive integration tests focusing on:
  * Database operations and persistence
  * Complete user workflow scenarios
  * Dialogue state transitions and validation
  * Multi-user scenarios and data integrity
- Removed trivial unit tests, kept only meaningful business logic tests
- Added UserRepository, ListingRepository, and BotMessenger traits
- Integration tests verify end-to-end workflows with real database
- Total: 30 focused tests (4 unit + 8 integration + 18 validation tests)

Key integration test scenarios:
- Complete listing creation workflow with database persistence
- Listing update workflows with state validation
- Multi-user independent operations
- Error handling that preserves data integrity
- Business rule enforcement (e.g., no timing changes on live listings)

Framework ready for advanced mock-based testing when needed.

Cargo.toml

@@ -26,6 +26,7 @@ thiserror = "2.0.16"
 teloxide-core = "0.13.0"
 num = "0.4.3"
 itertools = "0.14.0"
+async-trait = "0.1"
 
 [dev-dependencies]
 rstest = "0.26.1"