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/0004_add_sumup_reference.py

@@ -0,0 +1,27 @@
+"""Add sumup_reference to orders"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0004_add_sumup_reference"
+down_revision = "0003_add_orders" 
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "orders",
+        sa.Column("sumup_reference", sa.String(length=255), nullable=True),
+    )
+    op.create_index(
+        "ix_orders_sumup_reference",
+        "orders",
+        ["sumup_reference"],
+        unique=False,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_orders_sumup_reference", table_name="orders")
+    op.drop_column("orders", "sumup_reference")