feat: initial shared library extraction

Contains shared infrastructure for all coop services:
- shared/ (factory, urls, user_loader, context, internal_api, jinja_setup)
- models/ (User, Order, Calendar, Ticket, Product, Ghost CMS)
- db/ (SQLAlchemy async session, base)
- suma_browser/app/ (csrf, middleware, errors, authz, redis_cacher, payments, filters, utils)
- suma_browser/templates/ (shared base layouts, macros, error pages)
- static/ (CSS, JS, fonts, images)
- alembic/ (database migrations)
- config/ (app-config.yaml)
- editor/ (Lexical editor Node.js build)
- requirements.txt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

models/tag_group.py

@@ -0,0 +1,52 @@
+from datetime import datetime
+from typing import List, Optional
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy import (
+    Integer,
+    String,
+    Text,
+    DateTime,
+    ForeignKey,
+    UniqueConstraint,
+    func,
+)
+from db.base import Base
+
+
+class TagGroup(Base):
+    __tablename__ = "tag_groups"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    slug: Mapped[str] = mapped_column(String(191), unique=True, nullable=False)
+    feature_image: Mapped[Optional[str]] = mapped_column(Text())
+    colour: Mapped[Optional[str]] = mapped_column(String(32))
+    sort_order: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
+    )
+
+    tag_links: Mapped[List["TagGroupTag"]] = relationship(
+        "TagGroupTag", back_populates="group", cascade="all, delete-orphan", passive_deletes=True
+    )
+
+
+class TagGroupTag(Base):
+    __tablename__ = "tag_group_tags"
+    __table_args__ = (
+        UniqueConstraint("tag_group_id", "tag_id", name="uq_tag_group_tag"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    tag_group_id: Mapped[int] = mapped_column(
+        ForeignKey("tag_groups.id", ondelete="CASCADE"), nullable=False
+    )
+    tag_id: Mapped[int] = mapped_column(
+        ForeignKey("tags.id", ondelete="CASCADE"), nullable=False
+    )
+
+    group: Mapped["TagGroup"] = relationship("TagGroup", back_populates="tag_links")