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>

models/magic_link.py

@@ -0,0 +1,25 @@
+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 MagicLink(Base):
+    __tablename__ = "magic_links"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    token: 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)
+    purpose: Mapped[str] = mapped_column(String(32), nullable=False, default="signin")
+    expires_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    used_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
+    ip: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    user_agent: Mapped[str | None] = mapped_column(String(256), nullable=True)
+
+    user = relationship("User", backref="magic_links")
+
+    __table_args__ = (
+        Index("ix_magic_link_token", "token", unique=True),
+        Index("ix_magic_link_user", "user_id"),
+    )