Pydantic deprecations, psycopg2-binary removal, update backend dockerfile

[spwoodcock]

• Fixes Usage of psycopg2-binary in auth-libs is an antipattern #9 by removing psycopg2-binary, and instead using the bundled asyncpg lib for alembic migrations (asyncpg is used throughout the rest of the code models etc.
• Also removed the deprecated class Config: usage for pydantic, as it was replaced with ConfigDict in pydantic v2 a long time ago (ready for v3 total removal).
• Also updated the backend dockerfile to better match production patterns in the field-tm model Dockerfile (better caching strategy, not using uv run in production, and a few other changes).