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.

alerting/Dockerfile

@@ -1,7 +1,7 @@
 # alerting never needs /proto (it talks to /api over plain HTTP, no gRPC),
 # so unlike api/ingest/search this build context is just alerting/ itself,
 # same shape as cli/Dockerfile:
-#   docker build -f alerting/Dockerfile -t sentry-alerting alerting/
+#   docker build -f alerting/Dockerfile -t cairnobs-alerting alerting/
 FROM golang:1.25-alpine AS builder
 WORKDIR /src
 COPY . .