Add content index: keywords, tags/categories, search, and XML sitemap

Ships Blog tags/categories, Internal search, and XML sitemap together as
one shared mechanism, plus a new meta keywords request, rather than three
separate ones - the "worth deciding together" call ISSUES.md made when
XML sitemap was first written.

Content stays in files. Per-page metadata is four Twig blocks in the
root layout, the same override mechanism already used for
title/description/og_* - keywords (rendered), tags/changefreq/priority
(not rendered, harvested only). App\ContentIndexer crawls every routable
page (Overlay::listPageDirs(), new) and pulls each block via
Renderer::renderForIndex() (new) calling Twig's own renderBlock() API,
not regex-parsing .twig source, so overrides and layout inheritance
resolve exactly like a real render. Rendered HTML is stripped and
indexed into a SQLite FTS5 table for search.

Off by default (content_index_enabled) - same posture as Matomo/admin
auth, since this is a real SQLite dependency plenty of sites won't want.
Verified true zero footprint when disabled: no data/novaconium.sqlite
gets created, all three consumer routes 404 like they don't exist.
content_index_auto (default true) reindexes lazily on first stale touch
of a consumer route, never on a normal page view; both that path and the
explicit `php novaconium/bin/index-content.php` share one reindex().

Two real bugs caught by testing, not review: a reentrancy bug where
/search's own sidecar calling ensureFresh() during the crawl triggered a
nested reindex() mid-transaction (fixed with a static re-entrancy guard),
and a wrong PDO constant in the search sidecar. Also fixed two unrelated
pre-existing bugs found while building this: an unescaped {{ }} in
admin/docs/sidecars that fataled Twig on any real render of that page,
and a stale "no database layer yet" claim there and in Lib\Input's
docblock, left over from before Lib\Db shipped.

Lib\Db's migrations_dir now accepts an ordered list of roots, not just
one path, so the content index's schema could ship as a framework
migration (novaconium/migrations/) without colliding with project
migrations in App/migrations/ - the two-root extension point flagged in
AGENTS.md when SQLite groundwork shipped. Migrations are tracked by path
relative to the repo root rather than bare filename so two roots with a
same-named file can't shadow each other.

New consumer routes: novaconium/pages/sitemap.xml/ and
novaconium/pages/search/ (framework defaults), App/pages/blog/tag/[tag]/
(project-owned, since blog/ is project content - the existing hand
-written post array in App/pages/blog/index.php is untouched). Added
tags to the 4 existing blog posts as a real demonstration.

Closes the Blog tags/categories, Internal search, and XML sitemap
backlog items in novaconium/ISSUES.md.

App/pages/blog/twig-syntax-guide/index.twig

@@ -6,6 +6,7 @@
 {% block description %}A tour of the Twig syntax used throughout this site — output, filters, control structures, inheritance, and a few gotchas.{% endblock %}
 
 {% block robots %}index, follow{% endblock %}
+{% block tags %}twig, reference{% endblock %}
 {% block canonical %}{{ request_path|default('/') }}{% endblock %}
 
 {% block og_type %}article{% endblock %}