Fix migration 0001_initial: double CREATE TYPE crashes every clean bootstrap

world_visibility and asset_type were each declared twice: once explicitly
via `.create(checkfirst=True)`, and again implicitly by SQLAlchemy when
create_table() compiled the inline postgresql.ENUM() column type (which
defaults create_type=True). On a genuinely empty database this raises
asyncpg.DuplicateObjectError mid-transaction and the migration never
completes -- verified this was true 100% of the time from a fresh
container, meaning the backend has never successfully bootstrapped from
scratch. Fixed by passing create_type=False on the inline column
declarations, since the type is already created explicitly above.

Also parameterized both services' host ports (POSTGRES_HOST_PORT /
REDIS_HOST_PORT, defaulting to the original 5432/6379) so concurrent
agent workspaces sharing this same docker-compose.yml can run isolated
databases without colliding on the host port.

backend/alembic/versions/0001_initial.py

@@ -69,7 +69,8 @@ def upgrade() -> None:
         sa.Column(
             "visibility",
             postgresql.ENUM(
-                "public", "private", "friends_only", name="world_visibility"
+                "public", "private", "friends_only", name="world_visibility",
+                create_type=False,
             ),
             nullable=True,
         ),
@@ -89,7 +90,7 @@ def upgrade() -> None:
             "asset_type",
             postgresql.ENUM(
                 "image", "audio", "video", "document", "model_3d", "other",
-                name="asset_type",
+                name="asset_type", create_type=False,
             ),
             nullable=False,
         ),