Security hardening: fix IDOR vulnerabilities, add rate limiting, SSRF protection, and encryption upgrades

- Fix ReconciliationController and BankSync LinkAccount/UpdateLinkedAccount IDOR (verify account ownership)
- Add ASP.NET Core rate limiting: strict on auth endpoints, global on all API routes
- Harden SSRF validation on Ollama URL (IPv6-mapped bypass, link-local, 0.0.0.0 blocking)
- Upgrade encryption from AES-CBC to AES-GCM with HKDF key derivation (backward-compatible decrypt)
- Add startup validation that rejects default JWT/encryption keys in Production
- Add security headers (X-Frame-Options, CSP, HSTS, Cache-Control, nosniff) to API and nginx
- Mask account numbers in API responses (show last 4 digits only)
- Add password strength validation (min 8 chars) and BCrypt work factor 12
- Hash refresh tokens (SHA-256) before database storage
- Set JWT ClockSkew to zero for immediate expiry enforcement
- Add file upload size limit (10 MB) and filename sanitization
- Cap transaction search PageSize to 200
- Fix FileWatcherService cross-user account matching in multi-user deployments
- Sanitize console.error calls to prevent token leakage in browser logs
- Parameterize raw SQL in SimpleFinConnectionMigration
- Make AuthService.GenerateAuthResponse fully async

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/nginx.conf

@@ -4,6 +4,13 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
+    # Security headers
+    add_header X-Frame-Options "DENY" always;
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+    add_header X-Permitted-Cross-Domain-Policies "none" always;
+    add_header Content-Security-Policy "default-src 'self'; script-src 'self' https://cdn.plaid.com; connect-src 'self' wss:; style-src 'self' 'unsafe-inline'; img-src 'self' data:; font-src 'self' data:;" always;
+
     location / {
         try_files $uri $uri/ /index.html;
     }