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-4-rbac-design.md

@@ -108,7 +108,7 @@ yet. Stated plainly so this doesn't read as more built than it is.
 
 ## Schema
 
-Lives in `/metadata` (`sentry_metadata`), alongside everything else from
+Lives in `/metadata` (`cairnobs_metadata`), alongside everything else from
 Phase 3, per `/docs/phase-4-isolation-design.md`'s existing schema
 additions (`tenants`, the `tenant_id` backfill on `alert_state`/
 `delivery_log`). New tables, continuing that migration sequence: