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/services/recaptcha.js

@@ -30,10 +30,10 @@ async function verifyRecaptcha(token) {
     return false;
   }
   
-  // If no secret key configured, log warning but pass
+  // If no secret key configured, fail closed
   if (!secretKey) {
-    console.warn('reCAPTCHA enabled but no secret key configured');
-    return true;
+    console.warn('reCAPTCHA enabled but no secret key configured — blocking request');
+    return false;
   }
   
   try {