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.ini

@@ -84,9 +84,9 @@ path_separator = os
 # output_encoding = utf-8
 
 # database URL.  This is consumed by the user-maintained env.py script only.
-# other means of configuring database URLs may be customized within the env.py
-# file.
-sqlalchemy.url = driver://user:pass@localhost/dbname
+# Left unset here: env.py falls back to Settings().postgres.dsn (ADR-0009),
+# and test fixtures may override it programmatically before invoking Alembic.
+# sqlalchemy.url =
 
 
 [post_write_hooks]