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/audit/audit.go

@@ -3,11 +3,11 @@
 // section. Two independent defenses back the "no update/delete path
 // from the application layer" requirement -- both verified against a
 // live Postgres, not just written: audit_writer (this package's own
-// Postgres role, via its own connection pool, never the shared `sentry`
+// Postgres role, via its own connection pool, never the shared `cairnobs`
 // role every other store uses) has only INSERT+SELECT grants, and a
 // BEFORE UPDATE OR DELETE trigger (metadata/migrations/0015-0016)
 // rejects the operation for *any* role, including the table owner --
-// confirmed live: even `sentry` cannot UPDATE a row without first
+// confirmed live: even `cairnobs` cannot UPDATE a row without first
 // disabling the trigger, a privileged operation distinct from ordinary
 // application access.
 //