feat(db): Alembic migration for users table (async env, DATABASE_URL)

Adds alembic/ (async env.py driven by DATABASE_URL) and the 0001 users-table
migration matching the model. enabled now has a DB-level server_default of false
(secure default even for non-ORM inserts). Verified upgrade/downgrade on sqlite.

alembic/script.py.mako

@@ -0,0 +1,23 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+revision = ${repr(up_revision)}
+down_revision = ${repr(down_revision)}
+branch_labels = ${repr(branch_labels)}
+depends_on = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    ${downgrades if downgrades else "pass"}