fix(docker): include migrations directory in backend image

The migrations/ directory was missing from the Dockerfile COPY instructions,
causing ModuleNotFoundError when app.main tries to import and run migrations
during startup. This caused production deployments to fail with:
  ModuleNotFoundError: No module named 'migrations'

The automatic migration runner in main.py (line 157) needs this directory to:
1. Detect database schema changes
2. Run pending migrations
3. Initialize default admin user

Fixes production deployment failure when containers start.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/Dockerfile

@@ -50,6 +50,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 COPY --from=builder /root/.local /root/.local
 
 COPY app ./app
+COPY migrations ./migrations
 COPY alembic ./alembic
 COPY alembic.ini .
 COPY init_admin.py .