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/(app)/members/members-client.tsx

@@ -241,13 +241,13 @@ export function MembersClient({ members: initialMembers, currentRole }: Props) {
 
         <div className="bg-card rounded border border-border overflow-hidden">
           <div className="overflow-x-auto">
-          <table className="w-full min-w-[600px]">
+          <table className="w-full">
             <thead>
               <tr className="border-b border-border">
                 <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">MEMBER</th>
                 <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">TIER</th>
-                <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">JOINED</th>
-                <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">LAST PAYMENT</th>
+                <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground hidden md:table-cell">JOINED</th>
+                <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground hidden lg:table-cell">LAST PAYMENT</th>
                 <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">STATUS</th>
                 {isAdmin(currentRole) && (
                   <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">ACTIONS</th>
@@ -280,11 +280,11 @@ export function MembersClient({ members: initialMembers, currentRole }: Props) {
                         {m.tier?.toUpperCase()}
                       </span>
                     </td>
-                    <td className="px-4 py-3 font-mono text-xs text-muted-foreground">
-                      {m.joined_at ? new Date(m.joined_at).toLocaleDateString('en-US', { month: 'short', year: 'numeric' }) : '—'}
+                    <td className="px-4 py-3 font-mono text-xs text-muted-foreground hidden md:table-cell">
+                      {m.joined_at ? new Date(m.joined_at).toLocaleDateString('en-US', { month: 'short', year: 'numeric' }) : '-'}
                     </td>
-                    <td className="px-4 py-3 font-mono text-xs text-muted-foreground">
-                      {(m.last_paid_at || m.last_payment_at) ? new Date((m.last_paid_at || m.last_payment_at)!).toLocaleDateString('en-US', { month: 'short', year: 'numeric' }) : '—'}
+                    <td className="px-4 py-3 font-mono text-xs text-muted-foreground hidden lg:table-cell">
+                      {(m.last_paid_at || m.last_payment_at) ? new Date((m.last_paid_at || m.last_payment_at)!).toLocaleDateString('en-US', { month: 'short', year: 'numeric' }) : '-'}
                     </td>
                     <td className="px-4 py-3">
                       {m.status === 'unverified' ? (