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/oauth_code.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+from datetime import datetime
+from sqlalchemy import String, Integer, DateTime, ForeignKey, func, Index
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from shared.db.base import Base
+
+
+class OAuthCode(Base):
+    __tablename__ = "oauth_codes"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    code: Mapped[str] = mapped_column(String(128), unique=True, index=True, nullable=False)
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    client_id: Mapped[str] = mapped_column(String(64), nullable=False)
+    redirect_uri: Mapped[str] = mapped_column(String(512), nullable=False)
+    expires_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    used_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    grant_token: Mapped[str | None] = mapped_column(String(128), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
+
+    user = relationship("User", backref="oauth_codes")
+
+    __table_args__ = (
+        Index("ix_oauth_code_code", "code", unique=True),
+        Index("ix_oauth_code_user", "user_id"),
+    )