feat: route GET/HEAD requests to an optional read replica

[lesnik512]

Closes #45.

Port of modern-python/fastapi-sqlalchemy-template#71.

What

• New DB_REPLICA_DSN setting (default empty). When set, GET/HEAD requests use a replica engine; every other method, and any resolution without a request, uses the primary. When unset, behaviour is unchanged and no second pool is created.
• app/ioc.py: database_engine (primary), database_replica_engine (AsyncEngine | None), and a request-scoped dynamic_engine chosen by choose_sa_engine. The session gets dynamic_engine via explicit kwargs.
• Tests override dynamic_engine with the rolled-back connection, so reads and writes still share one transaction.
• tests/test_db_routing.py covers method routing, no-request fallback, no-replica fallback, and building the replica engine from the DSN.

Design notes

• Session wired to dynamic_engine explicitly; both base engines are bound_type=None so modern-di does not raise a duplicate-type error.
• The replica provider returns None rather than aliasing the primary provider, so routing is testable by overriding each engine independently.
• app/resources/*.py is exempted from ruff's TC rules: modern-di evaluates creator annotations at runtime, and moving import litestar under TYPE_CHECKING breaks resolution.
• Tests build requests with RequestFactory().get("/") and set the scope method, since RequestFactory has no HEAD helper and ty rejects a raw dict scope.

Caveats

• Replica lag: a client that writes and immediately reads may see stale data.
• GET handlers must not write; they would hit the replica.