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/HTACCESS_TO_NGINX.md

@@ -0,0 +1,36 @@
+# `.htaccess` → nginx migration (item #6)
+
+> **Problem:** `public/admin/.htaccess` contained Apache-specific security
+> directives that nginx **silently ignores**. None of the rules were active
+> in production.
+
+---
+
+## Rules migrated into `nginx/posterg.conf`
+
+| Apache `.htaccess` rule | nginx equivalent | Location |
+|---|---|---|
+| `Header always set X-Frame-Options "SAMEORIGIN"` | `add_header X-Frame-Options "SAMEORIGIN" always;` | main `server` block (already present) |
+| `Header always set X-Content-Type-Options "nosniff"` | `add_header X-Content-Type-Options "nosniff" always;` | main `server` block (already present) |
+| `Header always set X-XSS-Protection "1; mode=block"` | **Intentionally omitted** — deprecated & counterproductive; see `SECURITY_HEADERS.md` | — |
+| `Header always set Referrer-Policy "strict-origin-when-cross-origin"` | `add_header Referrer-Policy "strict-origin-when-cross-origin" always;` | main `server` block (already present) |
+| `Header always set Content-Security-Policy "..."` | `add_header Content-Security-Policy "..." always;` | `/admin/` location block (**added**) |
+| `Options -Indexes` | `autoindex off;` | `/admin/` location block (**added**; nginx default is off, explicit for clarity) |
+| `<FilesMatch "^\."> Require all denied` | `location ~ /\.(?!well-known).*` deny | main `server` block (already present) |
+| `<FilesMatch "(composer\.(json\|lock)\|error\.log)$"> Require all denied` | `location ~* \.(md\|txt\|sql\|sh\|json\|gitignore)$` deny + `location ~* \.log$` deny | main `server` block (`log` rule **added**) |
+| `php_flag display_errors Off` | Handled by `config/bootstrap.php` (`ini_set('display_errors', '0')`) | PHP |
+| `php_flag log_errors On` | Handled by `config/bootstrap.php` (`ini_set('log_errors', '1')`) | PHP |
+| `php_value error_log error.log` | Handled by `config/bootstrap.php`; should use absolute path (item #9) | PHP |
+
+---
+
+## Status of `public/admin/.htaccess`
+
+The file is now **dead code** on this nginx server. It has been left in place
+(harmless) so it would still work if the project were ever tested behind Apache
+(e.g., `php -S` built-in server doesn't read it either). All security rules it
+previously attempted to set are now enforced by nginx directly.
+
+---
+
+*Added: 2026-02-08 — security item #6*