feat(database): setup alembic and create scenarios table

- Install alembic and asyncpg for database migrations
- Configure alembic for async SQLAlchemy 2.0
- Create initial migration for scenarios table:
  * UUID primary key with auto-generation
  * Status enum (draft, running, completed, archived)
  * JSONB tags with GIN index
  * Timestamps with auto-update trigger
  * Check constraints for name/region validation
  * Indexes on status, region, created_at
- Test database connection and migration

Task: DB-001, DB-002 complete

alembic/README

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+Generic single-database configuration.

alembic/env.py

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+import asyncio
+from logging.config import fileConfig
+
+from sqlalchemy import pool
+from sqlalchemy.engine import Connection
+from sqlalchemy.ext.asyncio import async_engine_from_config
+
+from alembic import context
+
+# this is the Alembic Config object, which provides
+# access to the values within the .ini file in use.
+config = context.config
+
+# Interpret the config file for Python logging.
+# This line sets up loggers basically.
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+# add your model's MetaData object here
+# for 'autogenerate' support
+# from myapp import mymodel
+# target_metadata = mymodel.Base.metadata
+target_metadata = None
+
+# other values from the config, defined by the needs of env.py,
+# can be acquired:
+# my_important_option = config.get_main_option("my_important_option")
+# ... etc.
+
+
+def run_migrations_offline() -> None:
+    """Run migrations in 'offline' mode.
+
+    This configures the context with just a URL
+    and not an Engine, though an Engine is acceptable
+    here as well.  By skipping the Engine creation
+    we don't even need a DBAPI to be available.
+
+    Calls to context.execute() here emit the given string to the
+    script output.
+
+    """
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def do_run_migrations(connection: Connection) -> None:
+    context.configure(connection=connection, target_metadata=target_metadata)
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_async_migrations() -> None:
+    """In this scenario we need to create an Engine
+    and associate a connection with the context.
+
+    """
+
+    connectable = async_engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+
+    await connectable.dispose()
+
+
+def run_migrations_online() -> None:
+    """Run migrations in 'online' mode."""
+
+    asyncio.run(run_async_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"}

alembic/versions/8c29fdcbbf85_create_scenarios_table.py

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+"""create scenarios table
+
+Revision ID: 8c29fdcbbf85
+Revises:
+Create Date: 2026-04-07 13:45:17.403252
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+
+# revision identifiers, used by Alembic.
+revision: str = "8c29fdcbbf85"
+down_revision: Union[str, Sequence[str], None] = None
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # Create uuid extension
+    op.execute('CREATE EXTENSION IF NOT EXISTS "uuid-ossp";')
+
+    # Create scenarios table - the enum type will be created automatically
+    op.create_table(
+        "scenarios",
+        sa.Column(
+            "id",
+            postgresql.UUID(as_uuid=True),
+            primary_key=True,
+            server_default=sa.text("uuid_generate_v4()"),
+        ),
+        sa.Column("name", sa.String(255), nullable=False),
+        sa.Column("description", sa.Text(), nullable=True),
+        sa.Column("tags", postgresql.JSONB(), server_default="[]"),
+        sa.Column(
+            "status",
+            sa.Enum(
+                "draft", "running", "completed", "archived", name="scenario_status"
+            ),
+            nullable=False,
+            server_default="draft",
+        ),
+        sa.Column("region", sa.String(50), nullable=False, server_default="us-east-1"),
+        sa.Column(
+            "created_at",
+            sa.TIMESTAMP(timezone=True),
+            server_default=sa.text("NOW()"),
+            nullable=False,
+        ),
+        sa.Column(
+            "updated_at",
+            sa.TIMESTAMP(timezone=True),
+            server_default=sa.text("NOW()"),
+            nullable=False,
+        ),
+        sa.Column("completed_at", sa.TIMESTAMP(timezone=True), nullable=True),
+        sa.Column("started_at", sa.TIMESTAMP(timezone=True), nullable=True),
+        sa.Column("total_requests", sa.Integer(), server_default="0", nullable=False),
+        sa.Column(
+            "total_cost_estimate",
+            sa.DECIMAL(12, 6),
+            server_default="0.000000",
+            nullable=False,
+        ),
+    )
+
+    # Add constraints
+    op.create_check_constraint(
+        "chk_name_not_empty",
+        "scenarios",
+        sa.func.char_length(sa.func.trim(sa.column("name"))) > 0,
+    )
+    op.create_check_constraint(
+        "chk_region_not_empty",
+        "scenarios",
+        sa.func.char_length(sa.func.trim(sa.column("region"))) > 0,
+    )
+
+    # Add indexes
+    op.create_index("idx_scenarios_status", "scenarios", ["status"])
+    op.create_index("idx_scenarios_region", "scenarios", ["region"])
+    op.create_index(
+        "idx_scenarios_created_at", "scenarios", ["created_at"], postgresql_using="brin"
+    )
+    op.create_index("idx_scenarios_tags", "scenarios", ["tags"], postgresql_using="gin")
+
+    # Create trigger for updated_at
+    op.execute("""
+        CREATE OR REPLACE FUNCTION update_updated_at_column()
+        RETURNS TRIGGER AS $$
+        BEGIN
+            NEW.updated_at = NOW();
+            RETURN NEW;
+        END;
+        $$ language 'plpgsql';
+    """)
+
+    op.execute("""
+        CREATE TRIGGER update_scenarios_updated_at
+            BEFORE UPDATE ON scenarios
+            FOR EACH ROW
+            EXECUTE FUNCTION update_updated_at_column();
+    """)
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # Drop trigger
+    op.execute("DROP TRIGGER IF EXISTS update_scenarios_updated_at ON scenarios;")
+    op.execute("DROP FUNCTION IF EXISTS update_updated_at_column();")
+
+    # Drop indexes
+    op.drop_index("idx_scenarios_tags", table_name="scenarios")
+    op.drop_index("idx_scenarios_created_at", table_name="scenarios")
+    op.drop_index("idx_scenarios_region", table_name="scenarios")
+    op.drop_index("idx_scenarios_status", table_name="scenarios")
+
+    # Drop table
+    op.drop_table("scenarios")
+
+    # Drop enum type
+    op.execute("DROP TYPE IF EXISTS scenario_status;")