Trial state of 29 Oct 2026: ATOMA reader setup, app dashboards, embedding notes

Baseline before the clean-up (PD-13): the PD-11 reader setup and the PD-12 app
dashboards as they ran on the local stack, uncommitted until now. Not deployed,
not independently reviewed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

petrolyst-insights/templates/head_custom_extra.html

@@ -0,0 +1,6 @@
+{#- Petrolyst: replaces Superset's empty head_custom_extra.html, which spa.html includes on every
+    page (petrolyst_reader.register puts this folder first). Readers get reader.css; its address
+    names its content, as browsers keep static files for a year. -#}
+{% if petrolyst_is_reader() %}
+  <link rel="stylesheet" href="{{ assets_prefix }}/static/assets/branding/reader.css?v={{ petrolyst_reader_css_version }}">
+{% endif %}