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/lib/components/NavSidebar.svelte

@@ -9,9 +9,10 @@
 		type CurrentSession,
 		type LocalSession
 	} from '$lib/api';
-	import { getTheme, setTheme, type Theme } from '$lib/theme.svelte';
+	import { getTheme, setTheme, isLight, type Theme } from '$lib/theme.svelte';
 	import { getDensity, toggleDensity } from '$lib/density.svelte';
-	import logo from '$lib/assets/logo-horizontal-dark.svg';
+	import logoDark from '$lib/assets/logo-horizontal-dark.svg';
+	import logoLight from '$lib/assets/logo-horizontal-light.svg';
 
 	let {
 		onOpenPalette,
@@ -84,7 +85,7 @@
 <aside class="sidebar" class:mobile-open={mobileOpen}>
 	<div class="brand">
 		<a href="/" class="brand-link" onclick={onCloseMobile}>
-			<img src={logo} alt="Cairn OBS" class="brand-logo" />
+			<img src={isLight() ? logoLight : logoDark} alt="Cairn OBS" class="brand-logo" />
 		</a>
 		<button type="button" class="close-mobile" onclick={onCloseMobile} aria-label="Close menu">✕</button>
 	</div>