feat: add inventory transaction audit trail (Phase 2)

Adds complete audit trail for all stock movements:
- InventoryTransaction model with transaction types (reserve, fulfill,
  release, adjust, set, import, return)
- Alembic migration for inventory_transactions table
- Transaction logging in order_inventory_service for all order operations
- Captures quantity snapshots, order references, and timestamps

Each inventory operation now creates a transaction record for
accountability and debugging.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

alembic/versions/o3c4d5e6f7a8_add_inventory_transactions_table.py

@@ -0,0 +1,144 @@
+"""Add inventory_transactions table
+
+Revision ID: o3c4d5e6f7a8
+Revises: n2c3d4e5f6a7
+Create Date: 2026-01-01
+
+Adds an audit trail for inventory movements:
+- Track all stock changes (reserve, fulfill, release, adjust, set)
+- Link transactions to orders for traceability
+- Store quantity snapshots for historical analysis
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = "o3c4d5e6f7a8"
+down_revision = "n2c3d4e5f6a7"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Create transaction type enum
+    transaction_type_enum = sa.Enum(
+        "reserve",
+        "fulfill",
+        "release",
+        "adjust",
+        "set",
+        "import",
+        "return",
+        name="transactiontype",
+    )
+
+    op.create_table(
+        "inventory_transactions",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("vendor_id", sa.Integer(), nullable=False),
+        sa.Column("product_id", sa.Integer(), nullable=False),
+        sa.Column("inventory_id", sa.Integer(), nullable=True),
+        sa.Column("transaction_type", transaction_type_enum, nullable=False),
+        sa.Column("quantity_change", sa.Integer(), nullable=False),
+        sa.Column("quantity_after", sa.Integer(), nullable=False),
+        sa.Column("reserved_after", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("location", sa.String(), nullable=True),
+        sa.Column("warehouse", sa.String(), nullable=True),
+        sa.Column("order_id", sa.Integer(), nullable=True),
+        sa.Column("order_number", sa.String(), nullable=True),
+        sa.Column("reason", sa.Text(), nullable=True),
+        sa.Column("created_by", sa.String(), nullable=True),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.func.now(),
+        ),
+        sa.ForeignKeyConstraint(["vendor_id"], ["vendors.id"]),
+        sa.ForeignKeyConstraint(["product_id"], ["products.id"]),
+        sa.ForeignKeyConstraint(["inventory_id"], ["inventory.id"]),
+        sa.ForeignKeyConstraint(["order_id"], ["orders.id"]),
+        sa.PrimaryKeyConstraint("id"),
+    )
+
+    # Create indexes
+    op.create_index(
+        "ix_inventory_transactions_id",
+        "inventory_transactions",
+        ["id"],
+    )
+    op.create_index(
+        "ix_inventory_transactions_vendor_id",
+        "inventory_transactions",
+        ["vendor_id"],
+    )
+    op.create_index(
+        "ix_inventory_transactions_product_id",
+        "inventory_transactions",
+        ["product_id"],
+    )
+    op.create_index(
+        "ix_inventory_transactions_inventory_id",
+        "inventory_transactions",
+        ["inventory_id"],
+    )
+    op.create_index(
+        "ix_inventory_transactions_transaction_type",
+        "inventory_transactions",
+        ["transaction_type"],
+    )
+    op.create_index(
+        "ix_inventory_transactions_order_id",
+        "inventory_transactions",
+        ["order_id"],
+    )
+    op.create_index(
+        "ix_inventory_transactions_created_at",
+        "inventory_transactions",
+        ["created_at"],
+    )
+    op.create_index(
+        "idx_inv_tx_vendor_product",
+        "inventory_transactions",
+        ["vendor_id", "product_id"],
+    )
+    op.create_index(
+        "idx_inv_tx_vendor_created",
+        "inventory_transactions",
+        ["vendor_id", "created_at"],
+    )
+    op.create_index(
+        "idx_inv_tx_type_created",
+        "inventory_transactions",
+        ["transaction_type", "created_at"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("idx_inv_tx_type_created", table_name="inventory_transactions")
+    op.drop_index("idx_inv_tx_vendor_created", table_name="inventory_transactions")
+    op.drop_index("idx_inv_tx_vendor_product", table_name="inventory_transactions")
+    op.drop_index(
+        "ix_inventory_transactions_created_at", table_name="inventory_transactions"
+    )
+    op.drop_index(
+        "ix_inventory_transactions_order_id", table_name="inventory_transactions"
+    )
+    op.drop_index(
+        "ix_inventory_transactions_transaction_type", table_name="inventory_transactions"
+    )
+    op.drop_index(
+        "ix_inventory_transactions_inventory_id", table_name="inventory_transactions"
+    )
+    op.drop_index(
+        "ix_inventory_transactions_product_id", table_name="inventory_transactions"
+    )
+    op.drop_index(
+        "ix_inventory_transactions_vendor_id", table_name="inventory_transactions"
+    )
+    op.drop_index("ix_inventory_transactions_id", table_name="inventory_transactions")
+    op.drop_table("inventory_transactions")
+
+    # Drop enum
+    sa.Enum(name="transactiontype").drop(op.get_bind(), checkfirst=True)