chore: prepare repository for open source contribution

[NewCoder3294]

Summary

• refresh public-facing README, security docs, contributing notes, and add a maintainer runbook
• enable/standardize OSS repo automation: Dependabot titles, current GitHub Actions, flat ESLint config, and missing labels
• resolve local dependency audit alerts by updating vulnerable packages and adding pnpm overrides for patched transitive versions
• harden URL fetching in HLS and camera validation paths, sanitize SMS fallback logs, and clean CodeQL-reported dead code/test patterns

Cleanup notes

• Deleted remote branches that were already merged into main.
• This PR supersedes the open Dependabot PRs for npm and GitHub Actions updates.
• Dependabot alerts and automated security fixes are now enabled for the repository.

Verification

• pnpm typecheck
• pnpm lint (passes with 3 existing hook warnings)
• pnpm test
• pnpm audit --audit-level=moderate
• NEXT_PUBLIC_SUPABASE_URL=https://placeholder.supabase.co NEXT_PUBLIC_SUPABASE_ANON_KEY=placeholder_anon_key_for_build_only pnpm build