feat(db): Alembic migration for users table (async env, DATABASE_URL)

Adds alembic/ (async env.py driven by DATABASE_URL) and the 0001 users-table
migration matching the model. enabled now has a DB-level server_default of false
(secure default even for non-ORM inserts). Verified upgrade/downgrade on sqlite.

alembic/env.py

@@ -0,0 +1,54 @@
+"""Alembic migration environment (async, driven by DATABASE_URL)."""
+
+import asyncio
+import os
+from logging.config import fileConfig
+
+from alembic import context
+from sqlalchemy.ext.asyncio import create_async_engine
+
+from intervals_mcp_server.db.models import Base
+
+config = context.config
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = Base.metadata
+
+
+def _database_url() -> str:
+    url = os.environ.get("DATABASE_URL") or config.get_main_option("sqlalchemy.url")
+    if not url:
+        raise RuntimeError("DATABASE_URL is not set")
+    return url
+
+
+def run_migrations_offline() -> None:
+    context.configure(
+        url=_database_url(),
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+        compare_type=True,
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def _run(connection) -> None:
+    context.configure(connection=connection, target_metadata=target_metadata, compare_type=True)
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_migrations_online() -> None:
+    engine = create_async_engine(_database_url())
+    async with engine.connect() as connection:
+        await connection.run_sync(_run)
+    await engine.dispose()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    asyncio.run(run_migrations_online())