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.

search/src/config.rs

@@ -67,7 +67,7 @@ impl Config {
                 .split(',')
                 .map(str::to_string)
                 .collect(),
-            redpanda_topic: getenv("REDPANDA_TOPIC", "sentry.logs.raw"),
+            redpanda_topic: getenv("REDPANDA_TOPIC", "cairnobs.logs.raw"),
             index_path: PathBuf::from(getenv("INDEX_PATH", "/var/lib/cairnobs-search/index")),
             offsets_path: PathBuf::from(getenv(
                 "OFFSETS_PATH",

search/src/consumer.rs

@@ -23,7 +23,7 @@ use crate::tenants::ActiveTenantTracker;
 /// `TestTenantIDHeaderKeyConstantsMatch` does on the Go side.
 const TENANT_ID_HEADER_KEY: &str = "tenant_id";
 
-/// Reads the same `sentry.logs.raw` topic ingest's ClickHouse-writer
+/// Reads the same `cairnobs.logs.raw` topic ingest's ClickHouse-writer
 /// consumer reads, as an independent consumer group in spirit (its own
 /// offset tracking, own failure domain) even though rskafka doesn't speak
 /// Kafka's broker-side consumer-group protocol -- see offsets.rs. One