* feat: accept pdf, images and office files
* refactor: rename FileConverter to FileParser, simplify file retain API
- Rename engine/converters/ → engine/parsers/, FileConverter → FileParser,
ConverterRegistry → FileParserRegistry, MarkitdownConverter → MarkitdownParser
- Rename env var HINDSIGHT_API_FILE_CONVERTER → HINDSIGHT_API_FILE_PARSER
- Remove async/document_tags params from FileRetainRequest (always async now)
- Add retain_files() to Python Hindsight client and retainFiles() to TypeScript client
- Add sample.pdf to doc examples for working file upload demonstrations
- Update test_file_retain.py to use new parser names and always-async behavior
- Fix Go client missing os import in api_files.go
- Simplify postgresql.py storage to minimal schema
* fix: update rust CLI tests to use is_supported_file instead of is_text_file
* fix: patch Go api_files.go to add missing 'os' import after generation
* fix: insert 'os' import after 'net/url' in api_files.go patch for correct position
* chore: regenerate OpenAPI spec and clients (converter→parser description update)
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