Split TODO.md into todo/ folder by topic (completed tasks removed)

todo/03-system-cache.md

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+# System Page Caching — Database-Backed Status Cache
+
+## Problem
+
+The admin system page (`/admin/system.php`) runs expensive operations on every load:
+- `systemctl` subprocess calls (~4 checks × ~100ms each)
+- `curl` HTTP self-check (~200-500ms)
+- `disk_total_space()`/`disk_free_space()` (fast but unnecessary per-request)
+- Log file `tail` + `filesize` + `filemtime` (I/O bound)
+- Nginx config file reading
+
+## Solution: `system_cache` table + background refresh
+
+- [ ] **Add `system_cache` table** to schema: `CREATE TABLE system_cache (key TEXT PRIMARY KEY, value TEXT NOT NULL, updated_at INTEGER NOT NULL)`
+- [ ] **Add migration** `storage/migrations/007_system_cache.sql`
+- [ ] **Add `SystemCache` class** (`src/SystemCache.php`) with methods:
+  - `get(string $key, int $maxAgeSec = 60): ?array`
+  - `set(string $key, array $data): void`
+  - `isStale(string $key, int $maxAgeSec = 60): bool`
+- [ ] **Refactor `system.php` status section**:
+  1. Check `SystemCache::get('system_status', 120)` — 2-minute TTL
+  2. If cache hit → render from cache, show "mis en cache il y a X sec" label
+  3. If cache miss → run checks, store in cache, render
+  4. Add `?refresh=1` GET param to force-bypass cache
+- [ ] **Refactor `system.php` log sections** — avoid re-reading on every tab switch; only read the active tab's log
+- [ ] **Cache disk info** separately with 5-minute TTL: `SystemCache::get('disk_info', 300)`
+- [ ] **Cache PHP info** separately with 1-hour TTL: `SystemCache::get('php_info', 3600)`