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.

enterprise/cmd/enterprise-api/main.go

@@ -17,7 +17,7 @@
 // keeps running plain api/cmd/api, unchanged; a real multi-tenant
 // deployment runs this one instead.
 //
-// Both Helm (deploy/helm/sentry/templates/api.yaml vs
+// Both Helm (deploy/helm/cairnobs/templates/api.yaml vs
 // enterprise-api.yaml) and docker-compose.yml (COMPOSE_PROFILES) now
 // make this the deployment-topology choice, not just a binary sitting
 // unused alongside api's -- see this repo's CLAUDE.md. `search`'s write