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/social/meta_base.html

@@ -0,0 +1,54 @@
+{# social/meta_base.html — common, non-conflicting head tags #}
+{# Expected context:
+   site: { title, url, logo, default_image, twitter_site, fb_app_id, description? }
+   request: Quart request (for canonical derivation)
+   robots_override: optional string ("index,follow" / "noindex,nofollow")
+#}
+
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+
+{# Canonical #}
+{% set _site_url = site().url.rstrip('/') if site and site().url else '' %}
+{% set canonical = (
+    request.url if request and request.url
+    else (_site_url ~ request.path if request and _site_url else _site_url or None)
+) %}
+
+{# Robots: allow override; default to index,follow #}
+<meta name="robots" content="{{ robots_override if robots_override is defined else 'index,follow' }}">
+
+{# Theme & RSS #}
+<meta name="theme-color" content="#ffffff">
+{% if _site_url %}
+  <link rel="alternate" type="application/rss+xml"
+        title="{{ site().title if site and site().title else 'RSS' }}"
+        href="{{ _site_url }}/rss.xml">
+{% endif %}
+
+{# JSON-LD: Organization & WebSite are safe on all pages (don't conflict with BlogPosting) #}
+{% set org_jsonld = {
+  "@context": "https://schema.org",
+  "@type": "Organization",
+  "name": site().title if site and site().title else "",
+  "url": _site_url if _site_url else None,
+  "logo": site().logo if site and site().logo else None
+} %}
+<script type="application/ld+json">
+{{ org_jsonld | tojson }}
+</script>
+
+{% set website_jsonld = {
+  "@context": "https://schema.org",
+  "@type": "WebSite",
+  "name": site().title if site and site().title else "",
+  "url": _site_url if _site_url else canonical,
+  "potentialAction": {
+    "@type": "SearchAction",
+    "target": (_site_url ~ "/search?q={query}") if _site_url else None,
+    "query-input": "required name=query"
+  }
+} %}
+<script type="application/ld+json">
+{{ website_jsonld | tojson }}
+</script>