test(postgres): allow Testcontainers to override the migration sqlalchemy.url

alembic/env.py previously always overwrote sqlalchemy.url from
Settings().postgres.dsn, which made it impossible for a test fixture to
point Alembic at a Testcontainers-managed database. Now env.py only
sets it when unset, and a new integration suite runs `alembic upgrade
head` against a real Postgres container per ADR-0016 (no create_all()).

alembic/env.py

@@ -19,10 +19,13 @@ if config.config_file_name is not None:
     fileConfig(config.config_file_name)
 
 # Application models' MetaData, used for 'autogenerate' support. The database
-# URL is likewise sourced from application settings, not alembic.ini, so both
-# migrations and the app read the same env-derived configuration (ADR-0009).
+# URL is likewise sourced from application settings by default, so both
+# migrations and the app read the same env-derived configuration (ADR-0009) —
+# unless a caller (e.g. a test fixture pointing at a Testcontainers database)
+# has already set sqlalchemy.url on this Config before invoking Alembic.
 target_metadata = Base.metadata
-config.set_main_option("sqlalchemy.url", Settings().postgres.dsn)
+if not config.get_main_option("sqlalchemy.url"):
+    config.set_main_option("sqlalchemy.url", Settings().postgres.dsn)
 
 
 def run_migrations_offline() -> None: