Add Cucumber BDD testing with JUnit 5 and comprehensive database configuration

## Summary
- Migrated from JUnit 4 to JUnit 5 (Jupiter 5.11.4)
- Added Cucumber BDD framework (7.23.0) with Russian-language feature files
- Implemented comprehensive database configuration system
- Created extensive documentation and tutorials

## Changes

### BDD Testing
- Added Cucumber dependencies and JUnit Platform Suite integration
- Created Russian-language feature files for student management and database connection testing
- Implemented step definitions with English method names and Russian annotations
- Created RunCucumberTest.java with JUnit 5 @suite configuration

### Database Configuration
- Implemented three-tier configuration: environment variables → db.properties → defaults
- Added properties-maven-plugin for loading db.properties
- Created db.properties.example template
- Updated step definitions to use environment variable fallbacks

### Documentation
- Created BDD_TESTING.md - comprehensive guide to Cucumber BDD
- Created DATABASE_CONFIG.md - detailed database configuration guide
- Created JDBC_TUTORIAL.md - complete JDBC tutorial
- Updated POSTGRESQL_TUTORIAL.md and README.md with new content

### JUnit 5 Migration
- Upgraded all JUnit dependencies to 5.11.4
- Updated assertion method signatures (assertEquals parameter order)
- Replaced @RunWith with @suite and @IncludeEngines
- Fixed duplicate junit-jupiter-api dependency
- Added JaCoCo plugin version (0.8.12)

### Test Code
- Created PostgresJDBCPreparedStatement.java - SQL injection protection examples
- Created PostgresJDBCWithPool.java - HikariCP connection pool examples
- Updated PostgresJDBC.java with improved structure

### Cleanup
- Removed .idea directory (IDE files)
- Removed 000_intro.md (moved to README)

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

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

Author: stden

000_intro.md

@@ -0,0 +1,6 @@
+# Переменные окружения для подключения к базе данных
+# Этот файл не должен попадать в систему контроля версий
+
+DB_URL=jdbc:postgresql://localhost:5432/postgres
+DB_USER=postgres
+DB_PASSWORD=123