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/Dockerfile.api

@@ -2,7 +2,7 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
-RUN apt-get update && apt-get install -y --no-install-recommends libpq-dev && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y --no-install-recommends libpq-dev tesseract-ocr tesseract-ocr-eng && rm -rf /var/lib/apt/lists/*
 RUN pip install --no-cache-dir --upgrade pip
 
 COPY requirements.txt .