fix(deploy): include alembic in API Docker image

Adds alembic.ini and alembic/ directory to the production API image so
alembic upgrade head can run in-cluster as an init container.

Also carries migration 003 (make hashed_password nullable) from PR #66.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

alembic/versions/003_make_users_hashed_password_nullable.py

@@ -0,0 +1,26 @@
+"""Make users.hashed_password nullable.
+
+Better-Auth inserts users without hashed_password (passwords live in the
+accounts table). This column is now purely optional.
+
+Revision ID: 003_make_users_hashed_password_nullable
+Revises: 002_better_auth_tables
+Create Date: 2026-03-30
+"""
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "003_make_users_hashed_password_nullable"
+down_revision = "002_better_auth_tables"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.alter_column("users", "hashed_password", existing_type=sa.String(255), nullable=True)
+
+
+def downgrade() -> None:
+    op.alter_column("users", "hashed_password", existing_type=sa.String(255), nullable=False)