Finish the Cairn OBS rename through services, docs, and assets

The rename commit before this one covered module paths and the obvious
user-facing strings; this is the rest of it -- the places where "sentry"
was a default value, a filename, or a picture rather than a word in a
sentence.

Defaults that changed: CLICKHOUSE_DATABASE (sentry -> cairnobs),
POSTGRES_DATABASE (sentry_metadata -> cairnobs_metadata), and
POSTGRES_USERNAME (sentry -> cairnobs), across api/alerting/ingest and
the enterprise binaries, plus the compose files and migrate scripts that
create those objects. These are *defaults*, so a deployment that sets
them explicitly is unaffected -- but any deployment relying on the old
defaults must have its environment updated before it picks this up, or
it will come up pointing at a database that doesn't exist.

Also: the light-mode logo variants (the dark ones existed alone, so the
landing page and sidebar rendered a dark mark on a light background),
regenerated favicons, and the docs/README/threat-model prose that still
said Sentry.

web/src/routes/+page.svelte

@@ -3,7 +3,9 @@
 	// /search (the old root; every prior link/shortcut/drill-down that
 	// used to point at "/" expecting the query page now points at
 	// "/search" explicitly, see NavSidebar/CommandPalette/drilldown.ts).
-	import logo from '$lib/assets/logo-stacked-dark.svg';
+	import logoDark from '$lib/assets/logo-stacked-dark.svg';
+	import logoLight from '$lib/assets/logo-stacked-light.svg';
+	import { isLight } from '$lib/theme.svelte';
 	import { Button } from '$lib/components/ui';
 
 	const shortcuts: { href: string; label: string; hint: string }[] = [
@@ -15,7 +17,7 @@
 </script>
 
 <main>
-	<img src={logo} alt="Cairn OBS" class="logo" />
+	<img src={isLight() ? logoLight : logoDark} alt="Cairn OBS" class="logo" />
 	<p class="tagline">
 		One query bar for filter/stats queries and free-text search across every host and service
 		you're shipping logs from.