Manual re-ingest + progress display + failure handling

[markgewhite]

Summary

• Added IngestResult dataclass with ingested/skipped/failed counts and failure details
• Added force mode to ingest_folder (re-ingests all files regardless of hash)
• Added per-file on_progress callback for progress tracking
• Added try/except per file — failed documents are skipped, logged, and summarised
• Added "Re-ingest Documents" action button in Chainlit UI
• Re-ingest shows document count, progress step, and detailed summary with expandable failure list

Closes #11

Test plan

• Force mode ignores existing file hashes
• Failed files are skipped without blocking remaining files
• Failures recorded with filename and error message
• Progress callback called per file
• Re-ingest action button triggers forced full re-ingestion
• Summary shows ingested/failed counts
• Failed documents listed in expandable detail section
• Full test suite: 99 tests pass

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