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.

transport/README.md

@@ -5,7 +5,7 @@ Redpanda for local development, plus the script that provisions the topic
 
 ## Topic naming contract
 
-`ingest` defaults to `REDPANDA_TOPIC=sentry.logs.raw` (see
+`ingest` defaults to `REDPANDA_TOPIC=cairnobs.logs.raw` (see
 `/ingest/internal/config`). `provision-topics.sh` defaults to the same
 name. These aren't wired together automatically — if you change one,
 change the other, or override `REDPANDA_TOPIC` consistently wherever