Fix snapshot metadata serialization (Issue #1744)

[kbssrikar7]

Summary

• allow dict/list metadata in SnapshotMetadataModel so Evidently UI no longer crashes when reports include rich metadata
• add stringify_metadata() helper and use it in both SQL/local storage to keep metadata comparable while preserving original payloads
• document Windows virtualenv activation in README
• add test_snapshot_metadata_with_complex_values to cover the UI API

Testing

• pytest tests/ui/test_app.py -k metadata