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)/payments/payments-client.tsx

@@ -197,12 +197,14 @@ export function PaymentsClient({ payments: initialPayments, members, integration
 
           <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">
-                  {['PLATFORM', 'AMOUNT', 'FROM / NOTE', 'DATE', 'MEMBER'].map(h => (
-                    <th key={h} className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">{h}</th>
-                  ))}
+                  <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">PLATFORM</th>
+                  <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">AMOUNT</th>
+                  <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground hidden md:table-cell">FROM / NOTE</th>
+                  <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground hidden sm:table-cell">DATE</th>
+                  <th className="px-4 py-3 text-left font-mono text-[10px] tracking-widest text-muted-foreground">MEMBER</th>
                 </tr>
               </thead>
               <tbody className="divide-y divide-border">
@@ -212,11 +214,11 @@ export function PaymentsClient({ payments: initialPayments, members, integration
                       {p.platform?.toUpperCase()}
                     </td>
                     <td className="px-4 py-3 font-sans text-sm font-medium">{'$'}{Number(p.amount).toFixed(2)}</td>
-                    <td className="px-4 py-3 font-mono text-xs text-muted-foreground max-w-[200px] truncate">
-                      {p.from_identifier}{p.from_note ? ` — "${p.from_note}"` : ''}
+                    <td className="px-4 py-3 font-mono text-xs text-muted-foreground max-w-[200px] truncate hidden md:table-cell">
+                      {p.from_identifier}{p.from_note ? ` - "${p.from_note}"` : ''}
                     </td>
-                    <td className="px-4 py-3 font-mono text-xs text-muted-foreground">
-                      {p.transaction_date ? new Date(p.transaction_date).toLocaleDateString('en-US', { month: 'short', day: 'numeric' }) : '—'}
+                    <td className="px-4 py-3 font-mono text-xs text-muted-foreground hidden sm:table-cell">
+                      {p.transaction_date ? new Date(p.transaction_date).toLocaleDateString('en-US', { month: 'short', day: 'numeric' }) : '-'}
                     </td>
                     <td className="px-4 py-3">
                       {p.link_status === 'linked' && p.space_members ? (