bundled: write Symfony cache + log to user data dir (AppImage is read-only)

Symfony defaults Kernel::getCacheDir/getLogDir to <project>/var/cache
and <project>/var/log. In bundled mode those resolve inside the
AppImage's FUSE mount (/tmp/.mount_<hash>/usr/share/<app>/symfony/) —
read-only. Migrations fail at startup with:

  Unable to create the "cache" directory
  (/tmp/.mount_<hash>/usr/share/<app>/symfony/var/cache/prod).

…and frankenphp's worker can't warm a cache either, so even after the
binary spawns, the app is in a half-working state (which probably also
explains the persistent Reconnecting banner the user reported — once
migrations fail the supervisor sets Offline; even a successful
re-probe of /healthz wouldn't recover from a half-warm state).

Two-part fix, framework-side seam + app-side override:

  1. BackendConnection.cpp (runMigrations + spawnChild): mkdir
     <m_dataDir>/var/{cache,log} and pass them as APP_CACHE_DIR /
     APP_LOG_DIR env vars. <m_dataDir> resolves to
     ~/.local/share/<app> via QStandardPaths::AppDataLocation, so
     it's user-writable.

  2. App Kernel.php (skeleton + todo): override getCacheDir /
     getLogDir to honour the env vars. Falls back to parent
     behaviour when unset (dev mode keeps writing to var/cache like
     normal).

Database file already lives at <m_dataDir>/var/data.sqlite, so the DB
side was fine. Caddy autosaves to ~/.config/caddy and TLS storage to
~/.local/share/caddy — both user-writable. Mercure ran in-memory
mode in earlier logs so no extra storage redirect needed there.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

framework/skeleton/symfony/src/Kernel.php

@@ -10,4 +10,16 @@ use Symfony\Component\HttpKernel\Kernel as BaseKernel;
 final class Kernel extends BaseKernel
 {
     use MicroKernelTrait;
+
+    // The bundled-mode supervisor sets APP_CACHE_DIR / APP_LOG_DIR to writable
+    // paths under the user data dir; the AppImage mount is read-only.
+    public function getCacheDir(): string
+    {
+        return $_SERVER['APP_CACHE_DIR'] ?? parent::getCacheDir();
+    }
+
+    public function getLogDir(): string
+    {
+        return $_SERVER['APP_LOG_DIR'] ?? parent::getLogDir();
+    }
 }