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)/proposals/[id]/page.tsx

@@ -92,7 +92,7 @@ export default async function ProposalDetailPage({ params }: { params: Promise<{
           <p className="font-mono text-[10px] tracking-widest text-muted-foreground uppercase mb-3">
             Tally
           </p>
-          <div className="grid grid-cols-4 gap-3 mb-4">
+          <div className="grid grid-cols-2 sm:grid-cols-4 gap-3 mb-4">
             <Tally label="Yes" value={proposal.outcome_yes} accent="text-primary" />
             <Tally label="No" value={proposal.outcome_no} accent="text-orange-600" />
             <Tally label="Abstain" value={proposal.outcome_abstain} accent="text-muted-foreground" />

app/(app)/proposals/[id]/proposal-actions.tsx

@@ -53,13 +53,13 @@ export function ProposalActions({
           <p className="font-mono text-[10px] tracking-widest text-muted-foreground uppercase mb-3">
             {myVote ? 'Update your vote' : 'Cast your vote'}
           </p>
-          <div className="grid grid-cols-4 gap-2 mb-3">
+          <div className="grid grid-cols-2 sm:grid-cols-4 gap-2 mb-3">
             {votePositions.map(p => (
               <button
                 key={p}
                 type="button"
                 onClick={() => setPosition(p)}
-                className={`font-mono text-xs tracking-widest uppercase py-2 rounded border transition ${
+                className={`font-mono text-xs tracking-widest uppercase py-2.5 rounded border transition ${
                   position === p
                     ? 'bg-primary text-white border-primary'
                     : 'border-border text-foreground hover:border-primary'