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.

ingest/internal/agentregistry/agentregistry.go

@@ -1,7 +1,7 @@
 // Package agentregistry is the Postgres-backed implementation of
 // grpcserver.AgentRegistry -- ingest's half of agent inventory/remote
 // config (see /docs/agent-management-design.md). Writes into the same
-// sentry_metadata Postgres api reads/writes from for the web UI's
+// cairnobs_metadata Postgres api reads/writes from for the web UI's
 // inventory and edit-config views (api/agents), the same shared-schema-
 // different-services shape alerting and api already use for dashboards/
 // alert_rules.