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.

hack/alert-load-test/README.md

@@ -14,7 +14,7 @@ go run . --count 500000
 
 # 2. Run a webhook-sink so the (never-firing, by design) rules have a
 #    valid notification target to point at:
-docker run -d --name sentry-webhook-sink --network sentry_default \
+docker run -d --name cairnobs-webhook-sink --network sentry_default \
   -p 9099:9099 -v $(pwd)/../webhook-sink:/src -w /src golang:1.25-alpine go run .
 
 # 3. Run the load test:

hack/alert-load-test/main.go

@@ -43,7 +43,7 @@ var (
 	pollInterval = flag.Duration("poll-interval", 5*time.Second, "how often to poll GET /rules while observing")
 	concurrency  = flag.Int("concurrency", 20, "concurrent rule-creation requests")
 	skipCleanup  = flag.Bool("no-cleanup", false, "leave the seeded rules/target in place after the run")
-	webhookURL   = flag.String("webhook-url", "http://sentry-webhook-sink:9099/", "notification target URL -- default assumes a webhook-sink container reachable on the compose network")
+	webhookURL   = flag.String("webhook-url", "http://cairnobs-webhook-sink:9099/", "notification target URL -- default assumes a webhook-sink container reachable on the compose network")
 )
 
 func main() {