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>

containers.py

@@ -0,0 +1,20 @@
+"""
+Generic container concept — replaces hard-wired post_id FKs
+with container_type + container_id soft references.
+"""
+from __future__ import annotations
+
+
+class ContainerType:
+    PAGE = "page"
+    # Future: GROUP = "group", MARKET = "market", etc.
+
+
+def container_filter(model, container_type: str, container_id: int):
+    """Return SQLAlchemy filter clauses for a container reference."""
+    return [model.container_type == container_type, model.container_id == container_id]
+
+
+def content_filter(model, content_type: str, content_id: int):
+    """Return SQLAlchemy filter clauses for a content reference (e.g. CalendarEntryContent)."""
+    return [model.content_type == content_type, model.content_id == content_id]