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.

docs/phase-1-runbook.md

@@ -54,7 +54,7 @@ docker compose logs search
 ```
 
 You should see "search gRPC server listening" and rskafka connecting to
-all of `sentry.logs.raw`'s partitions. If you see nothing at all, check
+all of `cairnobs.logs.raw`'s partitions. If you see nothing at all, check
 `RUST_LOG=info` is set on the `search` service in `docker-compose.yml`.
 
 ### A2. Generate a log line and confirm both query paths agree