feat(ocr): Add memory management and auto-restart for OCR stability

OCR Service improvements:
- Single worker ThreadPoolExecutor to prevent parallel OCR memory accumulation
- Semaphore to ensure only one OCR operation at a time
- Explicit numpy array cleanup after each OCR step
- Forced garbage collection after every OCR request
- Memory threshold check (2500MB) with pre-processing GC
- Memory usage logging before/after processing

Backend auto-restart:
- Add run-with-restart.sh wrapper script for uvicorn
- Auto-restart on crash (OOM, etc.) with max 10 restarts
- Reset restart counter if process runs >60s (stable)
- Graceful exit on SIGINT/SIGTERM
- Update start-prod.sh and start-test.sh to use wrapper

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/run-with-restart.sh

@@ -0,0 +1,51 @@
+#!/bin/bash
+# Wrapper script that auto-restarts uvicorn on crash
+# Usage: ./run-with-restart.sh [port] [log_file]
+
+PORT=${1:-8000}
+LOG_FILE=${2:-/tmp/unified_backend.log}
+MAX_RESTARTS=10
+RESTART_COUNT=0
+RESTART_DELAY=3
+
+echo "[Backend Runner] Starting uvicorn with auto-restart (max $MAX_RESTARTS restarts)" | tee -a "$LOG_FILE"
+echo "[Backend Runner] Port: $PORT, Log: $LOG_FILE" | tee -a "$LOG_FILE"
+
+while [ $RESTART_COUNT -lt $MAX_RESTARTS ]; do
+    START_TIME=$(date +%s)
+    echo "" | tee -a "$LOG_FILE"
+    echo "[Backend Runner] $(date '+%Y-%m-%d %H:%M:%S') - Starting uvicorn (attempt $((RESTART_COUNT + 1))/$MAX_RESTARTS)..." | tee -a "$LOG_FILE"
+
+    # Run uvicorn - it will exit on crash (OOM, etc.)
+    uvicorn main:app --host 0.0.0.0 --port "$PORT" 2>&1 | tee -a "$LOG_FILE"
+    EXIT_CODE=$?
+
+    END_TIME=$(date +%s)
+    RUNTIME=$((END_TIME - START_TIME))
+
+    echo "[Backend Runner] $(date '+%Y-%m-%d %H:%M:%S') - uvicorn exited with code $EXIT_CODE after ${RUNTIME}s" | tee -a "$LOG_FILE"
+
+    # If it ran for more than 60 seconds, reset restart counter (was stable)
+    if [ $RUNTIME -gt 60 ]; then
+        RESTART_COUNT=0
+        echo "[Backend Runner] Process was stable (>${RUNTIME}s), resetting restart counter" | tee -a "$LOG_FILE"
+    else
+        RESTART_COUNT=$((RESTART_COUNT + 1))
+        echo "[Backend Runner] Quick crash detected, restart count: $RESTART_COUNT/$MAX_RESTARTS" | tee -a "$LOG_FILE"
+    fi
+
+    # Exit if user sent SIGINT (Ctrl+C) or SIGTERM
+    if [ $EXIT_CODE -eq 130 ] || [ $EXIT_CODE -eq 143 ]; then
+        echo "[Backend Runner] Received termination signal, exiting..." | tee -a "$LOG_FILE"
+        exit 0
+    fi
+
+    # Wait before restart
+    if [ $RESTART_COUNT -lt $MAX_RESTARTS ]; then
+        echo "[Backend Runner] Restarting in ${RESTART_DELAY}s..." | tee -a "$LOG_FILE"
+        sleep $RESTART_DELAY
+    fi
+done
+
+echo "[Backend Runner] Max restarts ($MAX_RESTARTS) reached. Giving up." | tee -a "$LOG_FILE"
+exit 1