fix(docker): remove out-of-context plugins COPY, fix app entrypoint and host binding

[samarthsugandhi]

Summary

Fixes three inconsistencies in the backend Dockerfile that prevented
docker compose up --build from working on a clean checkout.
Closes #381.

Changes

backend/Dockerfile

• Removed COPY plugins ./plugins — the plugins/ directory is
  outside the ./backend build context so this line caused a build
  failure. Plugins are already mounted correctly via the compose
  volume ./plugins:/app/plugins, making the COPY redundant.
• Fixed app entrypoint from secuscan.api:app to secuscan.main:app
  to match the actual FastAPI application module.
• Fixed --host 127.0.0.1 to --host 0.0.0.0 so uvicorn binds to
  all interfaces inside the container and is reachable from the host
  via the mapped port.

How to test

1. From the repository root run docker compose up --build
2. Confirm the backend image builds without error
3. Confirm the API is reachable at http://127.0.0.1:8081/api/v1/health

Notes

• docker-compose.yml is unchanged — the compose file was already
  correct with context: ./backend and the plugins volume mount
• No backend Python code changes

Closes #381

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[utksh1]

Looks good: fixes the backend Dockerfile entrypoint/host binding and removes the invalid plugins COPY for the current compose build context; checks are green.