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.

novaconium/config.php

@@ -48,9 +48,13 @@ return [
     // wholly replaced on a framework update — see
     // /admin/docs/getting-started's "Updating the framework" section) — a
     // top-level data/ directory, project-owned like App/, is the only safe
-    // place for it. migrations_dir is optional per connection; omit it to
-    // never run migrations against that connection (e.g. a read-only
-    // legacy database). NOTE: unlike every other key here, App/config.php
+    // place for it. migrations_dir is optional per connection (omit it to
+    // never run migrations against that connection, e.g. a read-only
+    // legacy database) and accepts either one path or an ordered list of
+    // roots — the default connection lists novaconium/migrations/ (framework
+    // -shipped schema, e.g. the content index — see /admin/docs/content-index)
+    // before App/migrations/ (project migrations), so framework migrations
+    // always apply first. NOTE: unlike every other key here, App/config.php
     // merges into db_connections one level deeper than a normal shallow
     // override — see the comment on Lib\Db::config() — so adding a second
     // connection there doesn't require repeating 'default'.
@@ -58,7 +62,10 @@ return [
         'default' => [
             'driver'         => 'sqlite',
             'path'           => __DIR__ . '/../data/novaconium.sqlite',
-            'migrations_dir' => __DIR__ . '/../App/migrations',
+            'migrations_dir' => [
+                __DIR__ . '/migrations',
+                __DIR__ . '/../App/migrations',
+            ],
         ],
     ],
 
@@ -73,4 +80,20 @@ return [
     // since a world-readable cached copy would otherwise permanently leak
     // the draft the first time an admin previewed it.
     'draft_routes' => [],
+
+    // Content index (see /admin/docs/content-index) — backs /sitemap.xml,
+    // /search, and blog tag browsing. Off by default: all three depend on
+    // SQLite (Lib\Db), a real dependency plenty of sites built on this
+    // framework won't want at all, the same reasoning that keeps Matomo
+    // and admin auth off by default above. When false, all three routes
+    // 404 exactly as if they didn't exist, and nothing ever touches
+    // Lib\Db because of this feature — no data/novaconium.sqlite gets
+    // created just because the code exists. content_index_auto only
+    // matters once enabled: true (the default) reindexes lazily,
+    // on-demand, the first time a stale index is actually needed (never on
+    // a normal page view); false disables that and leaves indexing
+    // entirely to `php novaconium/bin/index-content.php`, e.g. from a
+    // deploy step.
+    'content_index_enabled' => false,
+    'content_index_auto'    => true,
 ];