fix: address Shannon security assessment findings (37 vulnerabilities) (#254)

Remediate 4 Critical, 18 High, 10 Medium, and 5 Low vulnerabilities
identified in the Shannon security assessment (2026-03-20).

Critical fixes:
- Command injection via rsync SSH key path (INJ-VULN-01)
- Self-escalation to super_admin role (AUTHZ-VULN-11)
- Invite super_admin backdoor (AUTHZ-VULN-12)
- Handlebars SSTI in email templates (INJ-VULN-05)

Authentication hardening:
- Rate limit on share-link login (AUTH-VULN-01)
- X-Forwarded-For spoofing bypass (AUTH-VULN-02)
- reCAPTCHA fails closed when misconfigured (AUTH-VULN-03)
- Token revocation on admin/gallery logout (AUTH-VULN-04/05)
- Cookie Secure flag defaults true in production (AUTH-VULN-06)
- Remove JWT from admin login response body (AUTH-VULN-07)
- Timing-safe gallery slug validation (AUTH-VULN-09)
- Account lockout fails closed on DB error (AUTH-VULN-12)
- Session endpoint checks token revocation

Path traversal & file access:
- checksums endpoint path containment (INJ-VULN-03)
- manifest validate path containment (INJ-VULN-04)

XSS prevention:
- Block SVG data URIs in CSS sanitizer (XSS-VULN-01)
- Email preview iframe sandbox (XSS-VULN-02)
- SSR branding HTML escaping (XSS-VULN-03)
- User-Agent sanitization in feedback (XSS-VULN-04)

Authorization (IDOR):
- Event ownership middleware for all admin routes
- Cross-admin user profile read restriction (AUTHZ-VULN-10)

SSRF & infrastructure:
- Private IP validation for SMTP, S3, rsync hosts
- Replace inline JWT with standard adminAuth middleware
- CSRF Content-Type enforcement on mutating API endpoints
- CSP headers in nginx location blocks

Token revocation fix:
- Remove overly broad orWhere clause that invalidated all future tokens
- Allow empty-body POST requests (logout) in CSRF middleware

Co-authored-by: Paul Nothaft <[email protected]>

backend/src/middleware/ownership.js

@@ -0,0 +1,35 @@
+const { db } = require('../database/db');
+
+/**
+ * Middleware to enforce event ownership for non-super_admin users.
+ * Super admins bypass the check. Other admins can only access events they created.
+ */
+function requireEventOwnership(req, res, next) {
+  if (req.admin.roleName === 'super_admin') {
+    return next();
+  }
+
+  const eventId = req.params.eventId || req.params.id;
+  if (!eventId) {
+    return res.status(400).json({ error: 'Event ID is required' });
+  }
+
+  db('events')
+    .where('id', eventId)
+    .first()
+    .then((event) => {
+      if (!event) {
+        return res.status(404).json({ error: 'Event not found' });
+      }
+      // Allow access if: event has no owner (legacy/system), or admin owns it
+      if (event.created_by && event.created_by !== req.admin.id) {
+        return res.status(403).json({ error: 'Access denied' });
+      }
+      next();
+    })
+    .catch((err) => {
+      res.status(500).json({ error: 'Failed to verify ownership' });
+    });
+}
+
+module.exports = { requireEventOwnership };