Revert: Security fixes, response cache, and OpenRouter provider (#50)

[prasjaiswal]

Summary

• Reverts PR Add security fixes, response cache, and OpenRouter provider #50 ("Add security fixes, response cache, and OpenRouter provider")
• PR Fix CodeQL security vulnerabilities and lint issues #61 ("Fix CodeQL security vulnerabilities and lint issues") was a no-op (zero diff vs Add security fixes, response cache, and OpenRouter provider #50) so no separate revert needed

What is rolled back

• Response cache (response_cache.py)
• OpenRouter provider (entire package)
• Security hardening (SSRF validation, XML bomb protection, path injection fix, stack trace exposure fixes)
• Crawler site-spec docs and demo
• Frontend fixes (ChatSearch, parseSchema, useNlWeb)
• Workflow permissions in check.yml

Test plan

• Run make check in ask_api, crawler, and frontend
• Verify deployment starts successfully without reverted features

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