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.

docs/DATABASE_MISMATCH_VISUAL_COMPARISON.md

@@ -308,7 +308,7 @@ STACK TRACE:
 AFFECTED FILES:
 
 SQL Schema:
-  └─ Jellyfin.Server/sql/schema_init/create_database_schema.sql
+  └─ scripts/sql/schema_init/create_database_schema.sql
 	 Lines: 92-886 (31 tables with PascalCase names)
 
 C# Configuration: