feat(migrations): T11 setup Alembic and initial schema migration

- Initialize Alembic with alembic init alembic
- Configure alembic.ini to use DATABASE_URL from environment
- Configure alembic/env.py to import Base and models metadata
- Generate initial migration: c92fc544a483_initial_schema
- Migration creates all 4 tables: users, api_keys, api_tokens, usage_stats
- Migration includes all indexes, constraints, and foreign keys
- Test upgrade/downgrade cycle works correctly

Alembic commands:
- alembic upgrade head
- alembic downgrade -1
- alembic revision --autogenerate -m 'message'

Tests: 13 migration tests pass

alembic/script.py.mako

@@ -0,0 +1,28 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}