brand: rename product to "Read the Markets" (read.markets)

The product is now "Read the Markets" served at https://read.markets,
with the app at https://app.read.markets. "Cassandra" survives only as
the in-product AI persona (system prompt + "Ask Cassandra" chat label).

Centralised the brand in app/branding.py: BRAND_NAME, BRAND_SHORT,
DOMAIN, SITE_URL, APP_URL, EMAIL_FROM_DEFAULT. Jinja templates pull
{{ BRAND_NAME }} via globals registered in templates_env.py; Python
code reads branding.BRAND_NAME directly. The future-rename surface
is now a one-liner.

Updated: FastAPI app title, every page title (dashboard, news, log,
settings, upload, login, verify), header brand div, auth-card brands,
OTP email subject + HTML + plain-text bodies (incl. uppercase header
tag), OpenRouter X-Title + HTTP-Referer attribution headers, README.
Email tests now assert against branding.BRAND_NAME rather than the
literal string.

Internal identifiers deliberately kept on the legacy "cassandra" name
to avoid invalidating live sessions / advisory locks / configs:
cookies (cassandra_session, cassandra_pending) + itsdangerous salts,
MariaDB GET_LOCK keys, CASSANDRA_TOKEN env var, cassandra.css filename,
pyproject package name, localStorage prefs, outbound User-Agent strings.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/templates/settings.html

@@ -1,5 +1,5 @@
 {% extends "base.html" %}
-{% block title %}Cassandra · Settings{% endblock %}
+{% block title %}{{ BRAND_NAME }} · Settings{% endblock %}
 
 {% block main %}
 <section class="panel" style="grid-column: 1 / -1; max-width: 760px; margin: 0 auto;">