feat(klausur-service): Add Tesseract OCR, DSFA RAG, TrOCR, grid detection and vocab session store

New modules:
- tesseract_vocab_extractor.py: Bounding-box OCR with multi-PSM pipeline
- grid_detection_service.py: CV-based grid/table detection for worksheets
- vocab_session_store.py: PostgreSQL persistence for vocab sessions
- trocr_api.py: TrOCR handwriting recognition endpoint
- dsfa_rag_api.py + dsfa_corpus_ingestion.py: DSFA RAG corpus search

Changes:
- Dockerfile: Install tesseract-ocr + deu/eng language packs
- requirements.txt: Add PyMuPDF, pytesseract, Pillow
- main.py: Register new routers, init DB pools + Qdrant collections

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

klausur-service/Dockerfile

@@ -13,9 +13,12 @@ FROM python:3.11-slim
 
 WORKDIR /app
 
-# Install system dependencies
+# Install system dependencies (incl. Tesseract OCR for bounding-box extraction)
 RUN apt-get update && apt-get install -y --no-install-recommends \
     curl \
+    tesseract-ocr \
+    tesseract-ocr-deu \
+    tesseract-ocr-eng \
     && rm -rf /var/lib/apt/lists/*
 
 # Install Python dependencies