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/config/config.go

@@ -101,8 +101,8 @@ func Load() (Config, error) {
 		HTTPListenAddr: getenv("HTTP_LISTEN_ADDR", ":8082"),
 		Postgres: PostgresConfig{
 			Addr:     getenv("POSTGRES_ADDR", "localhost:5432"),
-			Database: getenv("POSTGRES_DATABASE", "sentry_metadata"),
-			Username: getenv("POSTGRES_USERNAME", "sentry"),
+			Database: getenv("POSTGRES_DATABASE", "cairnobs_metadata"),
+			Username: getenv("POSTGRES_USERNAME", "cairnobs"),
 			Password: getenv("POSTGRES_PASSWORD", ""),
 		},
 		OIDC: OIDCConfig{