chore: restructure firmware into subdirectory, add DDEV config

Move firmware files from repo root src/ into firmware/ to avoid
collision with Symfony's src/ PHP class directory. Add DDEV
config targeting PHP 8.4 / PostgreSQL 16 / nginx-fpm with
Imagick extension via docker-compose override.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.ddev/config.yaml

@@ -0,0 +1,12 @@
+name: pictureframe
+type: symfony
+docroot: public
+php_version: "8.4"
+webserver_type: nginx-fpm
+database:
+  type: postgres
+  version: "16"
+composer_version: "2"
+hooks:
+  post-start:
+    - exec: composer install --no-interaction 2>/dev/null || true