feat(auth): make users.password_hash nullable for OAuth-only accounts

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/alembic/versions/5bb055a1593e_users_password_hash_nullable.py

@@ -0,0 +1,47 @@
+"""users password_hash nullable
+
+Revision ID: 5bb055a1593e
+Revises: b1fad5ddf357
+Create Date: 2026-05-06 07:23:21.480252
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '5bb055a1593e'
+down_revision: Union[str, None] = 'b1fad5ddf357'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.alter_column(
+        "users",
+        "password_hash",
+        existing_type=sa.String(255),
+        nullable=True,
+    )
+
+
+def downgrade() -> None:
+    # NOTE: downgrade is non-trivial if any OAuth-only users exist.
+    # This downgrade fails fast in that case rather than corrupting data.
+    conn = op.get_bind()
+    null_count = conn.execute(
+        sa.text("SELECT COUNT(*) FROM users WHERE password_hash IS NULL")
+    ).scalar()
+    if null_count and null_count > 0:
+        raise RuntimeError(
+            f"Cannot downgrade: {null_count} OAuth-only users have NULL password_hash. "
+            "Set passwords or delete those rows before downgrading."
+        )
+    op.alter_column(
+        "users",
+        "password_hash",
+        existing_type=sa.String(255),
+        nullable=False,
+    )