refactor: align backend layout and supabase infra

Consolidate backend modules/tests under the backend package while syncing Supabase compose/env config and related plans.

backend/alembic/env.py

@@ -0,0 +1,90 @@
+from __future__ import annotations
+
+import asyncio
+import sys
+from logging.config import fileConfig
+from pathlib import Path
+from typing import TYPE_CHECKING, Any
+
+from alembic import context
+from sqlalchemy import pool
+from sqlalchemy.ext.asyncio import async_engine_from_config
+
+project_root = Path(__file__).resolve().parents[1]
+src_path = project_root / "src"
+if str(src_path) not in sys.path:
+    sys.path = [str(src_path), *sys.path]
+
+from core.config.settings import config  # noqa: E402
+from core.db.base import Base  # noqa: E402
+from models import Profile  # noqa: F401,E402
+
+if TYPE_CHECKING:
+    from sqlalchemy.engine import Connection
+
+alembic_config = context.config
+
+if alembic_config.config_file_name is not None:
+    fileConfig(alembic_config.config_file_name)
+
+target_metadata = Base.metadata
+
+
+def _get_database_url() -> str:
+    database_url = config.database_url
+    if not database_url:
+        raise RuntimeError(
+            "DATABASE_URL is not configured. Set SOCIAL_INFRA__SUPABASE__DATABASE_URL."
+        )
+    return database_url
+
+
+def _build_config() -> dict[str, Any]:
+    section = alembic_config.get_section(alembic_config.config_ini_section) or {}
+    return {**section, "sqlalchemy.url": _get_database_url()}
+
+
+def run_migrations_offline() -> None:
+    url = _get_database_url()
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        compare_type=True,
+        compare_server_default=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def _do_run_migrations(connection: "Connection" | Any) -> None:
+    context.configure(
+        connection=connection,
+        target_metadata=target_metadata,
+        compare_type=True,
+        compare_server_default=True,
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_migrations_online() -> None:
+    connectable = async_engine_from_config(
+        _build_config(),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    async with connectable.connect() as connection:
+        await connection.run_sync(_do_run_migrations)
+
+    await connectable.dispose()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    asyncio.run(run_migrations_online())