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.

storage/README.md

@@ -100,7 +100,7 @@ defaults so the two stay in sync out of the box):
 | `CLICKHOUSE_HTTP` | `http://localhost:8123` |
 | `CLICKHOUSE_USER` | `default` |
 | `CLICKHOUSE_PASSWORD` | (empty — override, see above) |
-| `CLICKHOUSE_DATABASE` | `sentry` |
+| `CLICKHOUSE_DATABASE` | `cairnobs` |
 
 There's also a `Dockerfile` (bash + curl baked in, `migrations/` copied in
 at build time) used by the root-level `docker-compose.yml` as a one-shot