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.

docs/agent-management-design.md

@@ -135,7 +135,7 @@ already configured for.
 ## Data model
 
 `metadata/migrations/0037_create_agents.sql`: one `agents` table, one
-row per `(tenant_id, host)`, in the same `sentry_metadata` Postgres
+row per `(tenant_id, host)`, in the same `cairnobs_metadata` Postgres
 dashboards/alert_rules already live in — not a new database, matching
 this project's established "shared schema, different services own
 different tables" shape. `tenant_id` defaults to `'default'` for
@@ -257,18 +257,18 @@ the process exits cleanly -- `pending_command` confirmed cleared and
 
 ## CLI surface (punch-list item 3)
 
-`sentryctl agents` (`cli/cmd/sentryctl/cmd_agents.go`), same list/get
+`cairnobsctl agents` (`cli/cmd/cairnobsctl/cmd_agents.go`), same list/get
 shape as `dashboards`/`alerts`, plus a `config` sub-subcommand
 (mirroring `dashboards permissions`) since an override has its own
 get/set/clear lifecycle distinct from the agent resource itself:
 
 ```
-sentryctl agents list|get <host>
-sentryctl agents config get <host>|clear <host>
-sentryctl agents config set <host> [--batch-max-size N] [--batch-flush-interval-ms N]
+cairnobsctl agents list|get <host>
+cairnobsctl agents config get <host>|clear <host>
+cairnobsctl agents config set <host> [--batch-max-size N] [--batch-flush-interval-ms N]
                             [--heartbeat-enabled true|false] [--heartbeat-interval-ms N]
                             [--journald-unit UNIT]
-sentryctl agents restart <host> [--yes]
+cairnobsctl agents restart <host> [--yes]
 ```
 
 `config set` is the one command with real logic beyond a thin HTTP