Create a randomized User-Agent utility

[Himish04]

Description:

Our fetchers in 'fetchers/' directory pulls from aggregators (Bigevent, dev.events, papercall.io, sessionize.com) plus the hardcoded flagship watchlist, using standard Python requests. Some of these calls have hardcoded Chrome User-Agents, others have none. Running this nightly via GitHub Actions will eventually get us blocked with 429 Too Many Requests by Cloudflare or similar.

Tasks:

• Create a shared utility module (e.g. utils/user_agents.py) containing an array of 10-15 modern, real-world browser User-Agent strings (Chrome, Firefox, Safari, Edge — mix of desktop/mobile)
• Export a function (e.g. get_random_user_agent()) that returns a random one using random.choice()
• Update every aggregator scraper (Bigevent, dev.events, papercall.io, sessionize.com, flagship watchlist fetcher) to use this function for outbound requests calls instead of hardcoded or missing User-Agent headers
• Leave Playwright/Browserless-driven scrapers alone if they already set their own UA — this utility is for the plain requests-based aggregator scrapers
• Confirm the nightly GitHub Actions workflow still runs cleanly after the change
• Document the utility in CONTRIBUTING so new aggregator scrapers use it by default

To claim: comment below so we don't get duplicate work.