feat: add http security headers to caddy config

Adds X-Frame-Options, X-Content-Type-Options, and Referrer-Policy
headers to all responses served by Caddy to mitigate clickjacking and
MIME-type sniffing.

.jules/sentinel.md

@@ -1,4 +1,4 @@
-## 2024-06-06 - Missing HTTP Security Headers
-**Vulnerability:** The Caddyfile configuration was missing standard HTTP security headers (X-Frame-Options, X-Content-Type-Options, Referrer-Policy), leaving the application susceptible to clickjacking, MIME-type sniffing, and sensitive data leakage via referrers.
-**Learning:** The application relies on Caddy as a reverse proxy, and by default, Caddy doesn't automatically add these specific security headers unless explicitly configured.
-**Prevention:** Ensure that reverse proxy configurations in new projects or updates to existing ones explicitly include essential HTTP security headers in a global scope to protect all served content.
+## 2025-02-28 - Caddy HTTP Security Headers Configuration
+**Vulnerability:** The application was missing basic security headers (X-Frame-Options, X-Content-Type-Options, Referrer-Policy), increasing exposure to clickjacking and MIME-sniffing attacks.
+**Learning:** Security posture isn't just about application-level code. The reverse proxy (Caddy) configuration serves as the first line of defense and is the correct layer to enforce global HTTP response headers.
+**Prevention:** Always verify that the reverse proxy layer enforces foundational HTTP security headers instead of relying solely on the application code to handle response security.

Caddyfile

@@ -7,7 +7,7 @@
   encode zstd gzip
 
   header {
-		X-Frame-Options "SAMEORIGIN"
+    X-Frame-Options "DENY"
     X-Content-Type-Options "nosniff"
     Referrer-Policy "strict-origin-when-cross-origin"
   }