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/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)