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>

browser/templates/macros/admin_nav.html

@@ -0,0 +1,21 @@
+{#
+  Shared admin navigation macro
+  Use this instead of duplicate _nav.html files
+#}
+
+{% macro admin_nav_item(href, icon='cog', label='', select_colours='', aclass=styles.nav_button) %}
+  {% import 'macros/links.html' as links %}
+  {% call links.link(href, hx_select_search, select_colours, True, aclass=aclass) %}
+    <i class="fa fa-{{ icon }}" aria-hidden="true"></i>
+    {{ label }}
+  {% endcall %}
+{% endmacro %}
+
+{% macro placeholder_nav() %}
+{# Placeholder for admin sections without specific nav items #}
+<div class="relative nav-group">
+  <span class="block px-3 py-2 text-stone-400 text-sm italic">
+    Admin options
+  </span>
+</div>
+{% endmacro %}