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.

app/layout.tsx

@@ -1,6 +1,5 @@
 import type { Metadata } from 'next'
 import { Geist, Geist_Mono } from 'next/font/google'
-import { Analytics } from '@vercel/analytics/next'
 import { Toaster } from 'sonner'
 import './globals.css'
 
@@ -10,7 +9,6 @@ const _geistMono = Geist_Mono({ subsets: ["latin"] });
 export const metadata: Metadata = {
   title: 'hackerspace.sh — Manage Your Space',
   description: 'The operating system for hackerspaces: members, tasks, projects, ops, payments, and comms.',
-  generator: 'v0.app',
   icons: {
     icon: [
       {
@@ -40,7 +38,6 @@ export default function RootLayout({
       <body className="font-sans antialiased">
         {children}
         <Toaster richColors position="bottom-right" />
-        <Analytics />
       </body>
     </html>
   )