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/Dockerfile

@@ -1,7 +1,7 @@
 # Build context must be the repo root (sentry/), since this needs both
 # api/ and proto/ (api now speaks gRPC to /search, using proto's checked-in
 # Go bindings via the `replace` directive in api/go.mod):
-#   docker build -f api/Dockerfile -t sentry-api .
+#   docker build -f api/Dockerfile -t cairnobs-api .
 
 FROM golang:1.25-alpine AS builder
 WORKDIR /src