feat: refactor database engine management with dual-backend support (…

[joda9]

Description

(OEP + local egon-data)

This commit introduces a comprehensive refactoring of how eDisGo manages database connections, enabling seamless switching between the OpenEnergyPlatform (OEP) and local egon-data PostgreSQL databases.

Engine as EDisGo property (edisgo/edisgo.py)

• Add lazy-initialized engine property to the EDisGo class, replacing the previous pattern of passing engine explicitly to every function call
• Support multiple initialization modes via new constructor parameters: engine, db_config_path, db_url, db_ssh, db_token
• Default to OEP engine when no configuration is provided
• Add __deepcopy__ method that excludes the unpicklable SQLAlchemy engine (contains _thread._local objects) and lets the copy lazily recreate its own connection

Optional engine parameter in all DB functions

Make the engine parameter optional (default=None) in all 33 functions across 8 modules that access the database. When engine is None, each function falls back to edisgo_object.engine. This maintains full backward compatibility — callers can still pass an explicit engine.

Modified modules:

• edisgo/io/timeseries_import.py (10 functions)
• edisgo/io/electromobility_import.py (4 functions)
• edisgo/io/dsm_import.py (3 functions)
• edisgo/io/heat_pump_import.py (2 functions)
• edisgo/io/generators_import.py (1 function)
• edisgo/io/storage_import.py (1 function)
• edisgo/network/heat.py (2 engine fallbacks)
• edisgo/network/timeseries.py (1 engine fallback)

Local DB schema resolution (edisgo/tools/config.py)

Extend import_tables_from_oep() to handle local egon-data databases:

• Auto-resolve schema mismatches: tables may reside in different schemas locally vs on OEP (e.g. egon_etrago_bus is in "grid" locally but imported via "supply" on OEP). The method now searches all schemas when a table is not found in the expected schema.
• Handle tables without primary keys (e.g. egon_map_zensus_grid_districts, egon_daily_heat_demand_per_climate_zone) by passing all columns as synthetic PK via __mapper_args__["primary_key"].

SSH tunnel lifecycle management (edisgo/io/db.py)

• ssh_tunnel() now returns tuple[str, SSHTunnelForwarder] instead of just the port string, so the server object is no longer lost
• engine() stores the SSH server as engine._ssh_server for later cleanup via server.stop()
• This fixes the "I/O operation on closed file" logging errors caused by orphaned paramiko keepalive threads writing to closed log handlers

Test infrastructure (tests/conftest.py)

• Add --runlocal flag to run all DB tests against both OEP and local egon-data database
• Add --egon-data-config flag for custom YAML config path
• Parametrize tests via db_engine fixture: each DB test runs as test_name[oep] and test_name[local] when --runlocal is active
• Add pytest_sessionfinish hook that disposes engines and stops SSH tunnels cleanly after all tests complete
• Suppress paramiko DEBUG keepalive logging (defense-in-depth)
• Migrate all 29 test methods across 10 test files from hardcoded pytest.engine to parametrized db_engine fixture

New files

• tests/io/test_db.py: 4 tests for SSH tunnel lifecycle (tunnel returns server, engine stores server, cleanup stops tunnel, OEP has no tunnel)
• egon-data.configuration.yaml: Template config file with placeholder credentials for local egon-data database connections

Fixes #450 #578

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• New and adjusted code is formatted using the pre-commit hooks
• New and adjusted code includes type hinting now
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The Read the Docs documentation is compiling correctly
• If new packages are needed, I added them the setup.py, and if needed the rtd_requirements.txt, the eDisGo_env.yml and the eDisGo_env_dev.yml.
• I have added new features to the corresponding whatsnew file