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/internal/chwriter/chwriter.go

@@ -16,7 +16,7 @@
 // chrunner.RunSQL resolves exactly one tenant per call from ctx (a
 // single request always belongs to one identity); WriteBatch resolves
 // per *record*, since one Kafka batch pulled off the shared
-// sentry.logs.raw topic can freely mix records from many different
+// cairnobs.logs.raw topic can freely mix records from many different
 // tenants -- see ingest/internal/grpcserver's doc comment for why
 // there's one shared topic, not topic-per-tenant.
 package chwriter