feat: extract shared infrastructure from shared_lib

Phase 1-3 of decoupling plan:
- Shared DB, models, infrastructure, browser, config, utils
- Event infrastructure (domain_events outbox, bus, processor)
- Structured logging
- Generic container concept (container_type/container_id)
- Alembic migrations for all schema changes

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

infrastructure/user_loader.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+from quart import session as qsession, g
+from sqlalchemy import select
+from sqlalchemy.orm import selectinload
+
+from shared.models.user import User
+from shared.models.ghost_membership_entities import UserNewsletter
+
+
+async def load_user_by_id(session, user_id: int):
+    """Load a user by ID with labels and newsletters eagerly loaded."""
+    stmt = (
+        select(User)
+        .options(
+            selectinload(User.labels),
+            selectinload(User.user_newsletters).selectinload(
+                UserNewsletter.newsletter
+            ),
+        )
+        .where(User.id == user_id)
+    )
+    result = await session.execute(stmt)
+    return result.scalar_one_or_none()
+
+
+async def load_current_user():
+    uid = qsession.get("uid")
+    if not uid:
+        g.user = None
+        g.rights = {"admin": False}
+        return
+
+    g.user = await load_user_by_id(g.s, uid)
+    g.rights = {l.name: True for l in g.user.labels} if g.user else {}