Monorepo: consolidate 7 repos into one

Combines shared, blog, market, cart, events, federation, and account
into a single repository. Eliminates submodule sync, sibling model
copying at build time, and per-app CI orchestration.

Changes:
- Remove per-app .git, .gitmodules, .gitea, submodule shared/ dirs
- Remove stale sibling model copies from each app
- Update all 6 Dockerfiles for monorepo build context (root = .)
- Add build directives to docker-compose.yml
- Add single .gitea/workflows/ci.yml with change detection
- Add .dockerignore for monorepo build context
- Create __init__.py for federation and account (cross-app imports)

shared/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")