fix(docker): install Alembic and run migrations via python -m (#12)

The backend image pip-installed requirements.txt but Alembic was never
declared, so entrypoint.sh failed with alembic: not found (exit 127).
Add alembic and psycopg so POSTGRES_URL migrations match alembic/env.py.
Invoke migrations with python -m alembic for consistent module resolution.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Jakub Doboš <kubo6472@users.noreply.github.com>

Author: kubo6472

backend/entrypoint.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
 set -e
 cd /app
-alembic upgrade head
+python -m alembic upgrade head
 exec uvicorn main:app --host 0.0.0.0 --port 8000

backend/requirements.txt

@@ -1,5 +1,7 @@
 fastapi>=0.115.0
 uvicorn[standard]>=0.32.0
+alembic>=1.13.0
+psycopg[binary]>=3.2.0
 sqlmodel>=0.0.22
 aiosqlite>=0.20.0
 fastapi-users>=14.0.0