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.

api/localauth/store_integration_test.go

@@ -35,7 +35,7 @@ func integrationStore(t *testing.T) *Store {
 		t.Skip("LOCALAUTH_TEST_POSTGRES_ADDR not set -- skipping live-Postgres integration test")
 	}
 	password := os.Getenv("LOCALAUTH_TEST_POSTGRES_PASSWORD")
-	dsn := fmt.Sprintf("postgres://sentry:%s@%s/sentry_metadata", password, addr)
+	dsn := fmt.Sprintf("postgres://cairnobs:%s@%s/cairnobs_metadata", password, addr)
 	pool, err := pgxpool.New(context.Background(), dsn)
 	if err != nil {
 		t.Fatalf("opening pool: %v", err)
@@ -285,3 +285,4 @@ func TestIntegrationGetPasswordHashByID(t *testing.T) {
 		t.Errorf("GetPasswordHashByID for an unknown ID: err = %v, want ErrNotFound", err)
 	}
 }
+