Complete the low-risk half of the Sentry -> Cairn OBS rebrand

Sweeps the references that carry no runtime coupling, and fixes one that
turned out to be a real bug rather than stale branding.

Docker network: sentry_default -> cairnobs_default across 23 runbook and
test-header `docker run` commands. Compose derives the network from the
directory name, so this lands together with renaming the working copy to
cairnobs/ -- the two are only correct as one change.

Stale references corrected: four Dockerfile "repo root (sentry/)"
headers; .env pointing at the long-renamed deploy/helm/sentry/ chart;
five Helm comments describing the topic as sentry.logs.raw when all four
code paths have defaulted to cairnobs.logs.raw for some time; an
absolute /home/john/Projects/sentry/ path in the operator's package doc,
now repo-relative; the hand-written Tenant CRD description in both of
its identical copies, whose Go source already said Cairn OBS.

Migration 0043 repoints the default tenant's data source. 0026 seeded it
with ('sentry', '/var/lib/sentry-search') to match what
api/internal/config then defaulted to; the rebrand later moved those
defaults to "cairnobs" and /var/lib/cairnobs-search without moving the
already-applied row, leaving the default tenant naming a ClickHouse
database nothing writes to. Scoped to the exact stale values so it is a
no-op on any deployment that set them deliberately. 0026's comment is
annotated as superseded; its applied SQL is untouched.

Deliberately not included: the gRPC wire packages (sentry.logs.v1,
sentry.agent.v1) and proto/sentry/ import paths, which cannot change
without a lockstep agent/server upgrade; the Helm chart's
sentry_metadata database and sentry role, which need a real Postgres
migration on existing deployments; and the compliance audit records in
docs/compliance/, which are a dated historical record.

go build, go vet, and go test pass for ingest and deploy/operator.

metadata/migrations/0026_seed_default_tenant_data_source.sql

@@ -1,7 +1,13 @@
 -- The 'default' tenant's single data source, pointing at the one
 -- ClickHouse database ("sentry") and Tantivy index every Phase 0-3
--- deployment already uses -- see api/internal/config's CLICKHOUSE_DATABASE
--- default and search's index path default.
+-- deployment already used at the time this was written -- then matching
+-- api/internal/config's CLICKHOUSE_DATABASE default and search's index
+-- path default.
+--
+-- SUPERSEDED: the Sentry -> Cairn OBS rebrand moved both of those
+-- defaults ("cairnobs" and /var/lib/cairnobs-search) without updating
+-- this already-applied seed. 0043 repoints the row. The SQL below is
+-- left exactly as applied -- do not edit it; fix forward instead.
 INSERT INTO data_sources (id, tenant_id, name, clickhouse_database_name, tantivy_index_path)
 SELECT '00000000-0000-0000-0000-000000000001', 'default', 'default', 'sentry', '/var/lib/sentry-search'
 WHERE NOT EXISTS (SELECT 1 FROM data_sources WHERE tenant_id = 'default')

metadata/migrations/0043_rebrand_default_data_source.sql

@@ -0,0 +1,38 @@
+-- Repoint the 'default' tenant's data source at the post-rebrand
+-- ClickHouse database and Tantivy index path.
+--
+-- 0026 seeded this row with ('sentry', '/var/lib/sentry-search'),
+-- correct at the time: it deliberately mirrored what api/internal/config
+-- then defaulted CLICKHOUSE_DATABASE to, and search's index path
+-- default. The Sentry -> Cairn OBS rebrand later moved both defaults --
+-- api/internal/config and ingest/internal/config now default to
+-- "cairnobs", and every index path in the tree is
+-- /var/lib/cairnobs-search (docker-compose's search-index-data mount,
+-- the Helm chart's search.yaml, and the per-tenant paths
+-- enterprise/cmd/enterprise-api builds) -- but the already-applied
+-- 0026 row did not move with them.
+--
+-- The result on any database where 0026 ran: the default tenant's data
+-- source names a ClickHouse database nothing writes to, and an index
+-- path nothing maintains. Editing 0026 in place would not fix those
+-- deployments, since it has already been recorded as applied -- hence a
+-- forward migration.
+--
+-- Scoped by the exact stale values rather than by tenant_id alone, so
+-- this is a no-op on:
+--   - fresh databases, where 0026's seed already reflects current
+--     defaults if it is ever re-run,
+--   - deployments that set CLICKHOUSE_DATABASE explicitly and corrected
+--     this row by hand.
+-- It will not clobber a deliberately-chosen database name.
+UPDATE data_sources
+SET clickhouse_database_name = 'cairnobs'
+WHERE tenant_id = 'default'
+  AND name = 'default'
+  AND clickhouse_database_name = 'sentry';
+
+UPDATE data_sources
+SET tantivy_index_path = '/var/lib/cairnobs-search'
+WHERE tenant_id = 'default'
+  AND name = 'default'
+  AND tantivy_index_path = '/var/lib/sentry-search';