feat: extract shared infrastructure from shared_lib

Phase 1-3 of decoupling plan:
- Shared DB, models, infrastructure, browser, config, utils
- Event infrastructure (domain_events outbox, bus, processor)
- Structured logging
- Generic container concept (container_type/container_id)
- Alembic migrations for all schema changes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

alembic/versions/0002_add_cart_items.py

@@ -0,0 +1,78 @@
+"""Add cart_items table for shopping cart"""
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = "0002_add_cart_items"
+down_revision = "0001_initial_schema"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "cart_items",
+        sa.Column("id", sa.Integer(), primary_key=True, autoincrement=True),
+
+        # Either a logged-in user *or* an anonymous session_id
+        sa.Column(
+            "user_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=True,
+        ),
+        sa.Column("session_id", sa.String(length=128), nullable=True),
+
+        # IMPORTANT: reference products.id (PK), not slug
+        sa.Column(
+            "product_id",
+            sa.Integer(),
+            sa.ForeignKey("products.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+
+        sa.Column(
+            "quantity",
+            sa.Integer(),
+            nullable=False,
+            server_default="1",
+        ),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("now()"),
+        ),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("now()"),
+        ),
+        sa.Column(
+            "deleted_at",
+            sa.DateTime(timezone=True),
+            nullable=True,
+        ),
+    )
+
+    # Indexes to speed up cart lookups
+    op.create_index(
+        "ix_cart_items_user_product",
+        "cart_items",
+        ["user_id", "product_id"],
+        unique=False,
+    )
+    op.create_index(
+        "ix_cart_items_session_product",
+        "cart_items",
+        ["session_id", "product_id"],
+        unique=False,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_cart_items_session_product", table_name="cart_items")
+    op.drop_index("ix_cart_items_user_product", table_name="cart_items")
+    op.drop_table("cart_items")