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/utils/passwordValidation.js

@@ -213,17 +213,17 @@ async function validatePasswordInContext(password, context, userData = {}) {
     // Base validation with gallery-specific options
     const result = validatePassword(password, galleryOptions);
     
-    // Override validation for common date formats
-    // Allow passwords like "04.07.2025", "04/07/2025", "04-07-2025"
-    const datePattern = /^\d{1,2}[.\/-]\d{1,2}[.\/-]\d{4}$/;
-    if (datePattern.test(password)) {
-      // Date format is valid for gallery passwords
-      return {
-        valid: true,
-        errors: [],
-        score: 2,
-        feedback: {}
-      };
+    // Only allow date-format passwords when complexity is 'simple'
+    if (complexityLevel === 'simple') {
+      const datePattern = /^\d{1,2}[.\/-]\d{1,2}[.\/-]\d{4}$/;
+      if (datePattern.test(password)) {
+        return {
+          valid: true,
+          errors: [],
+          score: 2,
+          feedback: {}
+        };
+      }
     }
     
     // Additional gallery-specific checks