Add dynamic orm session (read-write) #45

Port of modern-python/fastapi-sqlalchemy-template#30 (implemented in modern-python/fastapi-sqlalchemy-template#71) so the two templates stay in step.

Behaviour

• New DB_REPLICA_DSN setting, empty by default. Empty means no replica engine and no change in behaviour.
• When set, GET/HEAD requests use the replica engine; every other method, and any resolution without a request, uses the primary.

Shape

• app/resources/db.py: create_sa_engine(url), create_primary_sa_engine, create_replica_sa_engine (returns None when unset), close_sa_engine tolerating None, choose_sa_engine(primary_engine, replica_engine, request: litestar.Request | None = None).
• app/ioc.py: database_engine, database_replica_engine (both bound_type=None), request-scoped dynamic_engine; session wired to dynamic_engine via explicit kwargs.
• tests/conftest.py overrides dynamic_engine with the rolled-back connection.
• app/resources/*.py exempt from ruff TC rules, since modern-di evaluates creator annotations at runtime.
• Tests for method routing, no-request and no-replica fallbacks, and building the replica engine from the DSN.
• AGENTS.md updated.