feat(01-03): scaffold Alembic async config and author 0001_initial_schema migration

- backend/alembic.ini: script_location=migrations, sqlalchemy.url=%(DATABASE_MIGRATE_URL)s
- backend/migrations/env.py: async_engine_from_config + Base.metadata wiring;
  runtime os.environ.get("DATABASE_MIGRATE_URL") injection (alembic.ini interpolation
  does not read OS env directly)
- backend/migrations/versions/0001_initial_schema.py: creates all 11 tables in
  dependency order with correct FKs, indexes, and named constraints
- documents.user_id is nullable=True per D-03; Phase 2 adds NOT NULL
- Ends with GRANT + ALTER DEFAULT PRIVILEGES for docuvault_app (Pitfall 4)
- Also grants USAGE/SELECT on sequences (audit_log.id autoincrement)
- downgrade() drops all tables in reverse dependency order

backend/migrations/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"}