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>

platformio.ini

@@ -0,0 +1,7 @@
+[env:esp32dev]
+platform = espressif32
+board = esp32dev
+framework = arduino
+monitor_speed = 115200
+upload_port = /dev/ttyUSB0
+monitor_port = /dev/ttyUSB0