feat: brain PDF/image text extraction — pymupdf + tesseract OCR + vision API

- PDF: extracts selectable text via pymupdf, falls back to Tesseract OCR for scanned docs
- PDF: renders first page as screenshot thumbnail
- Images: Tesseract OCR for text extraction, OpenAI vision API fallback for photos
- Plain text files: direct decode
- All extracted text stored in extracted_text field for search/embedding
- Tested: PDF upload → text extracted → AI classified → searchable

New deps: pymupdf, pytesseract, Pillow
System dep: tesseract-ocr added to both Dockerfiles

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/brain/storage/a66d0767-a59e-4ede-878d-f74441e42830/original_upload/test-insurance.pdf

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+BT /F1 12 Tf 72 650 Td (Policyholder: Yusuf Suleman) Tj ET
+BT /F1 12 Tf 72 630 Td (Deductible: 500 dollars) Tj ET
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