Close search's active-tenant write-routing gap with a polled allowlist

search/src/consumer.rs's write-routing (built last pass) had no active-
tenant check at all: IndexRegistry.resolve() would open-or-create an
index directory for any syntactically-valid tenant_id, active or not --
unlike ClickHouse's chwriter.Registry (an active-tenants-only snapshot
built at enterprise-ingest startup) or the read side (gated by
searchclient.TenantChecker, a direct rbacstore query). search is AGPL
core with no Postgres access and no enterprise/ import allowed, so it
needed a network boundary instead -- the same shape ingest's
TenantResolver already uses against enterprise-auth, just Rust calling
Go instead of Go calling Go.

New GET /internal/active-tenants endpoint on enterprise-auth
(rbacstore.ListActiveTenantIDs + authhandler.handleActiveTenants),
gated on a RoleService Bearer credential -- server-to-server auth, the
same shape alerting presents to api, minted via the already-generic
enterprise-auth -mint-service-token search. search/src/tenants.rs's
ActiveTenantTracker polls it every 60s, blocking startup on the first
fetch succeeding (fail-closed cold start -- a control-plane outage at
boot must not silently accept every tenant_id) and keeping the last-
known-good set on any later refresh failure (a transient blip shouldn't
stop every tenant's indexing, only prevent the allowlist from growing/
shrinking until connectivity resumes). consumer.rs refuses any tagged
record whose tenant isn't in the polled set, before ever calling
resolve() -- IndexRegistry itself stays policy-free, matching the same
mechanism/policy split clickhousewriter.Writer vs. chwriter.Registry
already draws on the ClickHouse side.

Off unless ENTERPRISE_AUTH_URL/ENTERPRISE_AUTH_SERVICE_TOKEN are both
set (search/src/config.rs rejects exactly one being set) -- every
existing deployment is unaffected.

Verified with real HTTP round trips in this environment: tenants.rs's
tests exercise real reqwest requests (actual Authorization: Bearer
header, actual JSON parsing) against a hand-rolled dependency-free TCP
test server, including both fail-closed paths (rejected first fetch,
unreachable server). authhandler's new tests cover the credential-kind
distinction this endpoint exists to enforce -- a real human session,
even for a genuine Owner, must not satisfy a check meant for a service
identity.

One asymmetry remains, disclosed rather than fixed: chwriter.Registry's
snapshot still never refreshes (stale until enterprise-ingest restarts),
while ActiveTenantTracker's 60s poll gives Tantivy a materially tighter
staleness window. Neither is a live per-write check -- that would mean
a database/HTTP round trip per record, a throughput cost neither
implementation accepts -- so both have some staleness window by design;
the gap between the two windows is what's disclosed, not a claim either
is fully live.

docs/architecture.md

@@ -171,17 +171,21 @@ escape hatch is opaque to any compiler-injected filter.
   index. No "second binary" needed here, unlike ClickHouse — Tantivy has
   no grant system to gate a commercially-licensed credential behind, so
   `IndexRegistry` already lived directly in this AGPL-core binary, and
-  read/write just share it. One gap is disclosed rather than closed by
-  this change: unlike `chwriter.Registry` (an active-tenants-only
-  snapshot built at startup) and unlike the read side (gated by
-  `searchclient.TenantChecker`), this consumer's `resolve()` call has no
-  active-tenant check — the process has no Postgres access to check
-  against — so a still-valid-but-should-be-revoked ingest credential can
-  cause an index directory to be created for a tenant that's no longer
-  active. Narrow blast radius (an orphan, isolated, empty index, not
-  cross-tenant leakage, and only reachable with a real signed
-  credential), but real; see `search/src/registry.rs`'s doc comment on
-  `resolve`.
+  read/write just share it. The active-tenant gap this design left open
+  is now closed too: `search/src/tenants.rs`'s `ActiveTenantTracker`
+  polls a new `GET /internal/active-tenants` endpoint on
+  `enterprise-auth` — `search` has no Postgres access, so unlike
+  `chwriter.Registry`'s direct `rbacstore` query or the read side's
+  `searchclient.TenantChecker`, this needed a network call instead (the
+  same "network boundary, not import boundary" shape `ingest`'s
+  `TenantResolver` already uses against the same service, authenticated
+  with a RoleService credential like `alerting`↔`api`) — and
+  `consumer.rs` refuses any tagged record whose tenant isn't in the
+  polled allowlist. Off unless configured, fail-closed on the first
+  fetch, last-known-good on later refresh failures; see
+  `search/src/tenants.rs`'s doc comment for the full design and
+  `search/src/registry.rs`'s for how mechanism (index lifecycle) and
+  policy (the gate) responsibilities split.
 - `deploy/operator`'s `Tenant` CRD and `enterprise-api -provision-tenant`
   are now unified, deliberately lightweight: `-provision-tenant` stays
   the sole real actor (ClickHouse + `rbacstore`), and now also syncs its