feat: system page caching via SystemCache + system_cache SQLite table

Add a TTL-based cache for the expensive checks on the admin system page,
eliminating repeated systemctl subprocess calls (~4×~100ms), curl self-pings
(~200-500ms), disk_*_space() and PHP ini reads on every page load.

Changes:
- storage/migrations/007_system_cache.sql: new migration creating the
  system_cache table (key TEXT PK, value TEXT, updated_at INTEGER)
- storage/schema.sql: system_cache table added before pages table
- Applied migration to live storage/posterg.db
- src/SystemCache.php: new class with get/set/isStale/ageSeconds/invalidate;
  uses SQLite INSERT … ON CONFLICT upsert; no external dependencies
- src/Database.php: added getDatabasePath(): string accessor
- public/admin/system.php:
  - Bootstrap SystemCache at request start using the existing DB PDO handle
  - system_status: cached with 2-min TTL (systemctl + curl checks)
  - php_info: cached with 1-hour TTL (PHP ini values are runtime-constant)
  - disk_info: cached with 5-min TTL (total/free/used/pct tuple)
  - Logs section: unchanged — always reads live log tail per active tab
  - ?refresh=1 GET param invalidates all three cache keys before rendering
  - Status panel heading shows cache badge: '⚡ Cache — il y a Xs' (hit)
    or '⟳ Actualisé' (miss/fresh), styled via new .sys-cache-badge rules
- public/assets/css/system.css: .sys-cache-badge / --hit / --miss styles

public/assets/css/system.css

@@ -254,6 +254,29 @@
 }
 .sys-refresh-note a:hover { text-decoration: underline; }
 
+/* ── Cache freshness badges ──────────────────────────────────────────────── */
+.sys-cache-badge {
+    display: inline-block;
+    font-size: .68rem;
+    font-weight: 400;
+    font-family: ui-monospace, monospace;
+    padding: .1rem .45rem;
+    border-radius: 3px;
+    margin-left: .7rem;
+    vertical-align: middle;
+    line-height: 1.6;
+}
+.sys-cache-badge--hit {
+    background: rgba(255,193,7,.12);
+    color: #ffc107;
+    border: 1px solid rgba(255,193,7,.35);
+}
+.sys-cache-badge--miss {
+    background: rgba(76,175,80,.12);
+    color: #4caf50;
+    border: 1px solid rgba(76,175,80,.35);
+}
+
 /* ── Nginx config viewer ───────────────────────────────────────────────── */
 .nginx-source-badge {
     display: inline-block;