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_SCHEMA_MISMATCH_REPORT.md

@@ -453,7 +453,7 @@ public void ConfigureConventions(ModelConfigurationBuilder configurationBuilder)
    - `src/Jellyfin.Database/Jellyfin.Database.Implementations/ModelConfiguration/`
 
 ### Database Schema Files
-1. `Jellyfin.Server/sql/schema_init/create_database_schema.sql`
+1. `scripts/sql/schema_init/create_database_schema.sql`
    - Lines 92-886 (all table definitions)
 
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