Folder scanning + settings panel + auto-ingest

[markgewhite]

Summary

• Added scanner module for file discovery and MD5 hash computation
• Added ingest module for incremental ingestion with file-hash change detection (only new/changed files ingested)
• Added public load_file() to loader for single-file loading
• Added Chainlit settings panel with folder path input and recursive scan toggle
• Settings changes trigger re-scan and re-ingestion
• Auto-scan runs on app startup

Closes #10

Test plan

• Scanner discovers supported files (.pdf, .docx, .txt, .md) and excludes others
• Recursive/non-recursive scanning works correctly
• MD5 hash computed from file contents
• Unchanged files skipped during ingestion
• New files ingested with file_hash in metadata
• Relative paths stored in metadata
• Settings panel with folder path and recursive toggle
• Full test suite: 95 tests pass

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