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/models/kv.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+from datetime import datetime
+from sqlalchemy import String, Text, DateTime
+from sqlalchemy.orm import Mapped, mapped_column
+from shared.db.base import Base
+
+class KV(Base):
+    __tablename__ = "kv"
+    """Simple key-value table for settings/cache/demo."""
+    key: Mapped[str] = mapped_column(String(120), primary_key=True)
+    value: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, default=datetime.utcnow, onupdate=datetime.utcnow)