security: fix all HIGH priority items from TODO.SECURITY.md

Items resolved:
- #3 (HIGH): Move file uploads outside webroot to STORAGE_ROOT (/var/www/posterg/storage).
  Uploads were previously stored in public/admin/actions/data/ which is web-accessible.
- #4 (HIGH): Align file paths and add media.php controller.
  DB paths are now storage-relative (theses/YEAR/ID/file, covers/file).
  New public/media.php serves files with path-traversal jail, MIME allow-list,
  and proper caching headers. memoire.php and search.php updated to use /media.php?path=.
  Also fixed: cover images were never recorded in thesis_files (broken INSERT).
- #5 (HIGH): RateLimit::getClientIdentifier() now uses REMOTE_ADDR only.
  HTTP_X_FORWARDED_FOR and HTTP_CLIENT_IP are attacker-controlled headers that
  allowed unlimited rate-limit bypass by rotating spoofed IPs.
- #6 (HIGH): Port public/admin/.htaccess security rules to nginx/posterg.conf.
  Apache .htaccess directives are silently ignored by nginx; none were active.
  CSP added to /admin/ location block, .log file denial added globally,
  autoindex off made explicit. Documented in nginx/HTACCESS_TO_NGINX.md.

Supporting changes:
- config/bootstrap.php: add STORAGE_ROOT constant
- nginx/SECURITY_HEADERS.md: updated to reflect admin CSP and pending public CSP
- docs/TODO.SECURITY.md: items #3-6 moved to resolved; priority order updated

nginx/posterg.conf

@@ -74,6 +74,13 @@ server {
         # Rate limiting for admin
         limit_req zone=admin burst=5 nodelay;
 
+        # Content-Security-Policy — ported from public/admin/.htaccess which nginx ignores (item #6).
+        # Tighter policy for admin: no external sources, no object embedding.
+        add_header Content-Security-Policy "default-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; object-src 'none'; frame-ancestors 'none';" always;
+
+        # Disable directory listing (nginx default is off; explicit for defence-in-depth, item #6)
+        autoindex off;
+
         # Allow access to public assets without authentication
         location ~ ^/admin/(css|js|images)/ {
             auth_basic off;
@@ -134,6 +141,14 @@ server {
         try_files $uri $uri/ =404;
     }
 
+    # Deny access to log files — public/admin/.htaccess had this rule but Apache
+    # .htaccess directives are silently ignored by nginx (item #6).
+    location ~* \.log$ {
+        deny all;
+        access_log off;
+        log_not_found off;
+    }
+
     # Deny access to .htaccess files (if Apache's document root concurs with nginx's)
     location ~ /\.ht {
         deny all;