Include feature-flags.dev.json in backend Docker build context

The .dockerignore allow-list excluded feature-flags.dev.json, so COPY . /app
never copied it into the image. With FEATURE_FLAGS_FILE_OVERRIDE_PATH pointing
at it in backend.dev.env, the backend crashed on startup with FileNotFoundError.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: aapeliv

app/backend/.dockerignore

@@ -9,6 +9,7 @@
 !templates
 !alembic.ini
 !pyproject.toml
+!feature-flags.dev.json
 
 # It's not included into the final image, but is still needed here
 # for "--mount=type=bind"