Build per-tenant ClickHouse write-routing for ingest (Tantivy still deferred)

ingest tags every record with a tenant_id Kafka header (built previously),
but nothing consumed it to actually route the write. This closes that for
ClickHouse: enterprise/cmd/enterprise-ingest (a second binary, mirroring
enterprise-api) reuses ingest/consumer's own flush loop unchanged, with
enterprise/internal/chwriter.Registry -- a per-tenant clickhousewriter.Writer
registry -- swapped in as the writer. A batch pulled from the single shared
Redpanda topic can mix records from many tenants, so WriteBatch groups by
TenantID and dispatches each group to its own tenant's connection, fail-
closed on an empty or unrecognized tenant_id.

ingest/consumer and ingest/clickhousewriter move out of internal/ (same
reason api/internal/* moved earlier this phase: enterprise/ can't import
anything under another module's internal/). Their New() constructors now
take small local Config structs instead of ingest/internal/config types,
so enterprise/ doesn't need that import either.

Building this surfaced a real bug: tenantprovision.ProvisionClickHouse
only granted SELECT on a tenant's ClickHouse user, correct for chrunner's
read-only use but not enough for chwriter reusing the same credential to
write -- every real per-tenant write would have failed closed with a
permission error. Fixed by widening the grant to SELECT, INSERT; no
cross-tenant boundary is crossed by also allowing INSERT within a
tenant's own database.

Helm gates enterprise-ingest's Deployment on the same
ingest.requireTenantCredential flag that already gates tag validation --
write-routing is meaningless without tagging already being required, so
they're one decision, not two. docker-compose.yml's version is a
disclosed, weaker approximation: it can't achieve Helm's genuine
-mode=server/-mode=consumer split, so with the enterprise profile active
both ingest and enterprise-ingest independently consume every message
via different consumer groups -- harmless duplication for local
verification only.

Not built: Tantivy's independent Redpanda consumer (search/src/consumer.rs)
still doesn't read the tenant_id header at all -- every record still lands
in the one shared index regardless of tenant. Not run: the live-ClickHouse-
gated tests (chwriter's cross-tenant routing test, tenantprovision's INSERT
regression test) -- no Docker/database access in this environment; they're
correct Go that has never executed, disclosed as such in docs/security/
threat-model.md and docs/phase-4-runbook.md §14.

deploy/helm/sentry/values.yaml

@@ -76,7 +76,13 @@ ingest:
   # its own deliberate opt-in, not folded into enterprise.enabled
   # directly: turning it on requires every agent to already present a
   # valid ingest credential (`enterprise-auth
-  # -create-ingest-credential-tenant=<id>`) or be refused outright.
+  # -create-ingest-credential-tenant=<id>`) or be refused outright. Also
+  # controls per-tenant write-routing: true switches ingest.yaml's
+  # Deployment to -mode=server only and renders enterprise-ingest.yaml
+  # to take over consuming sentry.logs.raw, writing each tenant's
+  # records into their own ClickHouse database instead of the one
+  # shared table -- both flags gate together since write-routing is only
+  # meaningful once records actually carry a tenant_id to route on.
   requireTenantCredential: false
 
 search:
@@ -156,6 +162,15 @@ enterprise:
   apiImage:
     repository: sentry-enterprise-api
     tag: latest
+  # enterprise-ingest (templates/enterprise-ingest.yaml) -- only
+  # rendered when ingest.requireTenantCredential is also true (see that
+  # value's comment); takes over consuming sentry.logs.raw from
+  # ingest.yaml's own consumer once per-tenant write-routing is on. Same
+  # "repo root build context" reasoning as apiImage above -- see
+  # enterprise/cmd/enterprise-ingest/Dockerfile.
+  ingestImage:
+    repository: sentry-enterprise-ingest
+    tag: latest
   replicas: 1
   resources: {}
   # Leave empty to auto-generate (>= 32 bytes) and persist across