## ADDED Requirements

### Requirement: Configuration loading from appsettings.json
The system SHALL load the PostgreSQL connection string from `appsettings.json` using the `ConnectionStrings:DefaultConnection` configuration key.

#### Scenario: Successful configuration loading
- **WHEN** the application starts and `appsettings.json` exists with a valid `ConnectionStrings:DefaultConnection` entry
- **THEN** the system shall successfully load the connection string
- **AND** the connection string shall be available for database connections

#### Scenario: Missing connection string
- **WHEN** the application starts and `appsettings.json` is missing the `ConnectionStrings:DefaultConnection` entry
- **THEN** the system shall throw an `InvalidOperationException` with a clear error message indicating the missing configuration

#### Scenario: Invalid appsettings.json format
- **WHEN** the application starts and `appsettings.json` contains invalid JSON
- **THEN** the system shall throw a configuration exception indicating the JSON parsing error

### Requirement: Database query execution
The system SHALL provide a function that accepts a SQL query string and returns the first column of the first row as a string value.

#### Scenario: Successful query execution with single result
- **WHEN** a valid SQL query is executed that returns a single row with a single column
- **THEN** the system shall return the value as a string
- **AND** the database connection shall be properly opened and closed

#### Scenario: Query returning NULL value
- **WHEN** a SQL query is executed that returns a NULL value in the first column of the first row
- **THEN** the system shall return `null` instead of throwing an exception

#### Scenario: Query returning no rows
- **WHEN** a SQL query is executed that returns zero rows
- **THEN** the system shall return `null`

#### Scenario: Invalid SQL syntax
- **WHEN** a SQL query with invalid syntax is executed
- **THEN** the system shall throw a `PostgresException` with the PostgreSQL error details

#### Scenario: Database connection failure
- **WHEN** the database server is unreachable or credentials are invalid
- **THEN** the system shall throw a `PostgresException` or `NpgsqlException` with connection error details

### Requirement: Connection management
The system SHALL properly manage database connections using proper disposal patterns to prevent connection leaks.

#### Scenario: Connection properly disposed after query
- **WHEN** a query is executed successfully
- **THEN** the database connection shall be disposed after the query completes
- **AND** the connection shall be returned to the connection pool

#### Scenario: Connection disposed on exception
- **WHEN** a query execution throws an exception
- **THEN** the database connection shall still be properly disposed
- **AND** no connection leak shall occur

### Requirement: Static Service Initialization
The system SHALL provide a static method to initialize the database service with a connection string.

#### Scenario: Successful static initialization
- **WHEN** the `Initialize` method is called with a valid connection string
- **THEN** the static internal state shall be updated
- **AND** subsequent query calls shall use this connection string

### Requirement: Synchronous Database Query Execution
The system SHALL provide static functions that accept SQL query strings and return results synchronously.

#### Scenario: Successful synchronous query execution
- **WHEN** a valid SQL query is executed via a static method
- **THEN** the system shall return the result string synchronously
- **AND** the database connection shall be properly opened and closed synchronously

### Requirement: Static Configuration Access
The system SHALL provide static access to configuration properties through a dedicated service.

#### Scenario: Accessing connection string via static method
- **WHEN** the static `GetConnectionString()` method is called
- **THEN** it shall return the connection string from the internal static configuration
- **AND** it shall throw an `InvalidOperationException` if the configuration is missing the required key

### Requirement: Static Configuration Initialization
The system SHALL provide a way to initialize the static configuration state.

#### Scenario: Default static initialization
- **WHEN** the configuration service is first accessed without explicit initialization
- **THEN** it shall automatically load configuration from `appsettings.json`

#### Scenario: Explicit static initialization
- **WHEN** the `Initialize` method is called with a custom `IConfiguration` object
- **THEN** the static internal state shall be updated with the provided configuration