Competitor Price Monitoring Scraper

You are an expert Python developer specializing in ethical web scraping and data engineering.

Build a scheduled price-tracking scraper that monitors competitor products over time and detects significant price changes.

# Requirements

## Input
{{scraper-config}}

## Core Features

**Data Collection**
- Extract product identifier, current price, and timestamp on each run
- Append new records to a persistent CSV or SQLite historical store (never overwrite)
- Handle missing prices, unavailable products, and parsing errors gracefully

**Change Detection**
- Compare current prices against the most recent prior record for each product
- Flag any price change that exceeds the threshold specified in the config
- Calculate both absolute change (currency units) and percentage change

**Reporting**
- Generate a simple summary after each run showing:
  - Products with flagged changes, sorted by magnitude
  - Previous price → current price, absolute delta, percentage delta, direction (↑/↓)
  - Timestamp of comparison
  - Products unchanged or below threshold (brief count)
- Write report to console and append to a log file

**Ethical Compliance**
- Respect robots.txt for each target domain
- Enforce rate limiting: minimum 2-second delay between requests to the same domain (configurable)
- Set a realistic User-Agent header
- Include error handling for HTTP 429 (rate limit) and 403 (blocked) responses with exponential backoff
- Add comments explaining how to verify compliance with each site's Terms of Service before deployment

**Scheduling**
- Structure the script to run via cron, Task Scheduler, or APScheduler
- Include setup instructions for the chosen method in comments
- Log each run with success/failure status

## Technical Constraints
- Use requests + BeautifulSoup or httpx + selectolax for scraping
- Use pandas for CSV storage or sqlite3 for database storage
- Include requirements.txt with pinned versions
- Make file paths, storage format, and rate-limit parameters easily configurable (config file or environment variables)
- Write clean, maintainable code with docstrings

## Deliverables
1. Complete scraper script with scheduling capability
2. Configuration template (JSON or .env)
3. Setup instructions in comments or separate README block
4. Example output showing the change report format

Ensure the code is production-ready, well-commented, and includes error handling for common failure modes (network errors, HTML structure changes, disk space).