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/README.md

@@ -46,7 +46,7 @@ Environment variables (see `internal/config/config.go`):
 |---|---|---|
 | `HTTP_LISTEN_ADDR` | `:8080` | |
 | `CLICKHOUSE_ADDR` | `localhost:9000` | Native protocol port |
-| `CLICKHOUSE_DATABASE` / `_USERNAME` / `_PASSWORD` | `sentry` / `default` / `` | |
+| `CLICKHOUSE_DATABASE` / `_USERNAME` / `_PASSWORD` | `cairnobs` / `default` / `` | |
 | `SEARCH_GRPC_ADDR` | `localhost:50052` | Must match `/search`'s `GRPC_LISTEN_ADDR` |
 | `QUERY_TIMEOUT_SECONDS` | `30` | Per-request timeout |
 | `CORS_ALLOWED_ORIGIN` | `*` | Wide open by default since there's no auth yet; tighten together |
@@ -66,7 +66,7 @@ go test ./...
 
 ```sh
 # from the repo root, not api/
-docker build -f api/Dockerfile -t sentry-api .
+docker build -f api/Dockerfile -t cairnobs-api .
 ```
 
 ## Testing notes