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

src/SystemCache.php

@@ -0,0 +1,111 @@
+<?php
+
+/**
+ * SystemCache — thin TTL cache for admin system page checks.
+ *
+ * Stores JSON-encoded data blobs in the `system_cache` SQLite table.
+ * The table has a single schema:
+ *   key TEXT PRIMARY KEY, value TEXT NOT NULL, updated_at INTEGER NOT NULL
+ *
+ * Usage:
+ *   $cache = new SystemCache($pdo);
+ *
+ *   // Read (returns array or null if stale/missing)
+ *   $data = $cache->get('system_status', 120);
+ *
+ *   // Write
+ *   $cache->set('system_status', $myArray);
+ *
+ *   // Check freshness without reading value
+ *   if ($cache->isStale('disk_info', 300)) { ... }
+ *
+ *   // Force-invalidate a key (e.g. on ?refresh=1)
+ *   $cache->invalidate('system_status');
+ */
+class SystemCache
+{
+    private PDO $pdo;
+
+    public function __construct(PDO $pdo)
+    {
+        $this->pdo = $pdo;
+    }
+
+    /**
+     * Return cached data for $key if it is no older than $maxAgeSec seconds.
+     * Returns null when the entry is missing or stale.
+     */
+    public function get(string $key, int $maxAgeSec = 60): ?array
+    {
+        $stmt = $this->pdo->prepare(
+            'SELECT value, updated_at FROM system_cache WHERE key = ?'
+        );
+        $stmt->execute([$key]);
+        $row = $stmt->fetch(PDO::FETCH_ASSOC);
+
+        if (!$row) {
+            return null;
+        }
+
+        if ((time() - (int)$row['updated_at']) > $maxAgeSec) {
+            return null; // stale
+        }
+
+        $decoded = json_decode((string)$row['value'], true);
+        return is_array($decoded) ? $decoded : null;
+    }
+
+    /**
+     * Upsert $data (JSON-encoded) for $key with current timestamp.
+     */
+    public function set(string $key, array $data): void
+    {
+        $stmt = $this->pdo->prepare(
+            'INSERT INTO system_cache (key, value, updated_at)
+             VALUES (?, ?, ?)
+             ON CONFLICT(key) DO UPDATE SET value = excluded.value, updated_at = excluded.updated_at'
+        );
+        $stmt->execute([$key, json_encode($data), time()]);
+    }
+
+    /**
+     * Return true when the entry is missing or older than $maxAgeSec.
+     */
+    public function isStale(string $key, int $maxAgeSec = 60): bool
+    {
+        $stmt = $this->pdo->prepare(
+            'SELECT updated_at FROM system_cache WHERE key = ?'
+        );
+        $stmt->execute([$key]);
+        $row = $stmt->fetch(PDO::FETCH_ASSOC);
+
+        if (!$row) {
+            return true;
+        }
+
+        return (time() - (int)$row['updated_at']) > $maxAgeSec;
+    }
+
+    /**
+     * Return the age of the cached entry in seconds, or null if missing.
+     */
+    public function ageSeconds(string $key): ?int
+    {
+        $stmt = $this->pdo->prepare(
+            'SELECT updated_at FROM system_cache WHERE key = ?'
+        );
+        $stmt->execute([$key]);
+        $row = $stmt->fetch(PDO::FETCH_ASSOC);
+
+        return $row ? (time() - (int)$row['updated_at']) : null;
+    }
+
+    /**
+     * Delete a cached entry, forcing the next get() to re-compute.
+     */
+    public function invalidate(string $key): void
+    {
+        $stmt = $this->pdo->prepare('DELETE FROM system_cache WHERE key = ?');
+        $stmt->execute([$key]);
+    }
+}