Add PostgresSubprocessException and refactor database initialization logic

- Introduced PostgresSubprocessException to handle errors during PostgreSQL subprocess execution.
- Refactored PostgresDatabaseProvider to improve schema initialization script discovery.
- Enhanced error handling for psql command execution, including logging of output and errors.
- Implemented methods to find the schema initialization script and the psql executable path.

convert_sql_identifiers.ps1

@@ -1,7 +1,7 @@
 # PowerShell script to convert remaining SQL identifiers from quoted PascalCase to lowercase/snake_case
 # This script handles constraints, indexes, and foreign keys
 
-param([string]$filepath = "Jellyfin.Server/sql/schema_init/create_database_schema.sql")
+param([string]$filepath = "scripts/sql/schema_init/create_database_schema.sql")
 
 $content = Get-Content $filepath -Raw