Add Alembic migration scaffold

Author: anth-volk

.claude/skills/database-migrations.md

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+# Database Migration Guidelines
+
+## Overview
+
+This project uses Alembic for database migrations. API v1 still uses raw SQL
+initializers rather than ORM models, so Alembic target metadata is reflected
+from `policyengine_api/data/initialise_local.sql` by default.
+
+## Rules
+
+- Do not manually author Alembic operations for normal schema changes.
+- Generate migrations with `uv run alembic revision --autogenerate`.
+- Review generated migrations before applying them.
+- Keep SQL initializers and generated migrations aligned.
+- For pre-existing production databases, stamp the base revision before applying
+  new upgrade revisions.
+
+## Commands
+
+```bash
+uv run alembic revision --autogenerate -m "Description"
+uv run alembic upgrade head
+uv run alembic current
+uv run alembic history
+uv run alembic stamp <revision>
+```
+
+## API v1 Notes
+
+- Set `POLICYENGINE_ALEMBIC_DATABASE_URL` to the database SQLAlchemy URL Alembic
+  should connect to.
+- Set `POLICYENGINE_ALEMBIC_SCHEMA_SQL` when generating against a temporary
+  schema SQL file instead of the current initializer.
+- The base migration should be stamped in production because the tables already
+  exist there.

alembic.ini

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+# Alembic configuration for PolicyEngine API v1.
+
+[alembic]
+script_location = %(here)s/alembic
+file_template = %%(year)d%%(month).2d%%(day).2d_%%(rev)s_%%(slug)s
+prepend_sys_path = .
+path_separator = os
+output_encoding = utf-8
+
+# Overridden by alembic/env.py. For local generation, set
+# POLICYENGINE_ALEMBIC_DATABASE_URL explicitly.
+sqlalchemy.url = sqlite:///policyengine_api/data/policyengine.db
+
+[post_write_hooks]
+
+[loggers]
+keys = root,sqlalchemy,alembic
+
+[handlers]
+keys = console
+
+[formatters]
+keys = generic
+
+[logger_root]
+level = WARN
+handlers = console
+qualname =
+
+[logger_sqlalchemy]
+level = WARN
+handlers =
+qualname = sqlalchemy.engine
+
+[logger_alembic]
+level = INFO
+handlers =
+qualname = alembic
+
+[handler_console]
+class = StreamHandler
+args = (sys.stderr,)
+level = NOTSET
+formatter = generic
+
+[formatter_generic]
+format = %(levelname)-5.5s [%(name)s] %(message)s
+datefmt = %H:%M:%S

alembic/README

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+PolicyEngine API v1 Alembic migrations.
+
+This project does not currently use SQLAlchemy ORM models. Alembic
+autogenerate reflects target metadata from `policyengine_api/data/initialise_local.sql`
+or from the path in `POLICYENGINE_ALEMBIC_SCHEMA_SQL`.
+
+Use `alembic stamp` for pre-existing production databases before applying
+incremental migrations.

alembic/env.py

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+"""Alembic environment for PolicyEngine API v1 raw-SQL schema migrations."""
+
+from logging.config import fileConfig
+import importlib.util
+import os
+from pathlib import Path
+import sys
+
+from sqlalchemy import engine_from_config, pool
+
+from alembic import context
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+metadata_path = (
+    Path(__file__).parent.parent / "policyengine_api" / "data" / "alembic_metadata.py"
+)
+metadata_spec = importlib.util.spec_from_file_location(
+    "policyengine_api_alembic_metadata",
+    metadata_path,
+)
+if metadata_spec is None or metadata_spec.loader is None:
+    raise RuntimeError(f"Could not load Alembic metadata helper from {metadata_path}")
+metadata_module = importlib.util.module_from_spec(metadata_spec)
+metadata_spec.loader.exec_module(metadata_module)
+build_metadata_from_sql = metadata_module.build_metadata_from_sql
+
+
+config = context.config
+
+database_url = os.environ.get("POLICYENGINE_ALEMBIC_DATABASE_URL") or os.environ.get(
+    "DATABASE_URL"
+)
+if database_url:
+    config.set_main_option("sqlalchemy.url", database_url)
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+schema_sql_path = os.environ.get("POLICYENGINE_ALEMBIC_SCHEMA_SQL")
+target_metadata = build_metadata_from_sql(schema_sql_path)
+
+
+def _configure_context(connection=None, url: str | None = None) -> None:
+    options = {
+        "target_metadata": target_metadata,
+        "compare_type": False,
+        "compare_server_default": False,
+    }
+    if connection is not None:
+        context.configure(connection=connection, **options)
+    else:
+        context.configure(
+            url=url,
+            literal_binds=True,
+            dialect_opts={"paramstyle": "named"},
+            **options,
+        )
+
+
+def run_migrations_offline() -> None:
+    _configure_context(url=config.get_main_option("sqlalchemy.url"))
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    connectable = engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    with connectable.connect() as connection:
+        _configure_context(connection=connection)
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()

alembic/script.py.mako

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+"""${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"}