Fix API ModuleNotFoundError in dev compose

uv sync runs before the source is present in the image, so the local
package install is broken. Set PYTHONPATH=/app/src so Python finds
rehearsalhub directly from the mounted source volume — same approach
the worker Dockerfile already uses.

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

docker-compose.dev.yml

@@ -36,6 +36,7 @@ services:
       SECRET_KEY: ${SECRET_KEY:-replace_me_with_32_byte_hex_default}
       INTERNAL_SECRET: ${INTERNAL_SECRET:-replace_me_with_32_byte_hex_default}
       DOMAIN: ${DOMAIN:-localhost}
+      PYTHONPATH: /app/src
     ports:
       - "8000:8000"
     networks: