feat: open-source release prep, audit fixes, and deploy hardening

Production deploy completed (DigitalOcean Droplet, self-hosted Supabase,
Caddy + Let's Encrypt, GitHub Actions auto-deploy on push to main, daily
pg_dumpall backups). This commit lands the work that came out of the
deep audit pass and the open-source release prep.

Audit fixes:
- Webhook signing secret is now displayed in the settings UI with
  show/hide and copy-to-clipboard; the broken external docs.hackerspace.sh
  link is replaced with a link to the new local docs.
- Settings updateSpaceSettings parameter type now includes
  mission_statement so the field persists.
- members page enforces member_directory_visibility for non-admin viewers
  (admin_only, board_visible, members_visible).
- Mobile responsiveness pass: Projects Kanban grid, Members + Payments
  tables (column hiding on small screens), Proposal tally + voting
  buttons (2-col on mobile).

Open-source release scaffolding:
- LICENSE (MIT).
- CONTRIBUTING.md with dev setup, conventions, PR flow.
- SECURITY.md with disclosure policy.
- CHANGELOG.md.
- docs/WEBHOOKS.md with Node and Python signature verification examples.
- README.md rewritten for an open-source audience.

Removed v0/Vercel references:
- @vercel/analytics dependency removed.
- app/layout.tsx no longer imports Analytics, drops generator: 'v0.app'.
- Canonical deployment docs (docs/DEPLOYMENT.md, docs/README.md,
  docs/ARCHITECTURE.md, DATABASE.md, CLAUDE.md, README.md) now describe
  the self-hosted Droplet target only.
- e2e console-error check no longer filters [v0] noise.

Landing page:
- GitHub icon link to the public repo in the nav and footer.

CLAUDE.md

@@ -38,7 +38,7 @@ This file is the standing brief for any Claude session in this repository. Read
 ## Deployment rules
 
 - A fresh clone must produce a working app with no more than: clone, `pnpm install`, copy `.env.example` to `.env.local` with real values, run `scripts/schema.sql` in Supabase, `pnpm dev`.
-- A fresh production deploy (Vercel, DigitalOcean App Platform, Droplet) must be documented in `docs/DEPLOYMENT.md` step by step, no implicit knowledge required.
+- A fresh production deploy (self-hosted Supabase + app on a DigitalOcean Droplet) must be documented in `docs/DEPLOYMENT.md` step by step, no implicit knowledge required.
 - Any new environment variable must be added to `.env.example` in the same change that introduces it. Never let a deploy fail because an env var was only documented in a Slack thread.
 - The Dockerfile and docker-compose configuration in the repo are the source of truth for self-hosted deploys. Update them when the runtime requirements change.