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

storage/migrations/007_system_cache.sql

@@ -0,0 +1,8 @@
+-- Migration 007: Add system_cache table for admin system page caching
+-- Stores JSON-encoded status snapshots keyed by section with a TTL mechanism.
+
+CREATE TABLE IF NOT EXISTS system_cache (
+    key        TEXT    PRIMARY KEY,
+    value      TEXT    NOT NULL,
+    updated_at INTEGER NOT NULL
+);

storage/schema.sql

@@ -283,6 +283,12 @@ CREATE TABLE IF NOT EXISTS thesis_files (
 -- ============================================================================
 
 -- For managing editable static pages (charte, about, licenses, contact)
+CREATE TABLE IF NOT EXISTS system_cache (
+    key        TEXT    PRIMARY KEY,
+    value      TEXT    NOT NULL,
+    updated_at INTEGER NOT NULL
+);
+
 CREATE TABLE IF NOT EXISTS pages (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     slug TEXT NOT NULL UNIQUE, -- 'charte', 'about', 'licenses', 'contact'