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/h8f6c2d4e5a9_merge_heads.py

@@ -0,0 +1,23 @@
+"""merge heads
+
+Revision ID: h8f6c2d4e5a9
+Revises: c3d4e5f6a7b8, g7e5b1c3d4f8
+Create Date: 2026-02-11 00:00:00.000000
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = 'h8f6c2d4e5a9'
+down_revision = ('c3d4e5f6a7b8', 'g7e5b1c3d4f8')
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    pass
+
+
+def downgrade() -> None:
+    pass