Give ingest a real tenant identity (write-routing deferred, disclosed)

Ingest tenant-awareness was named "undesigned, not just unbuilt" across
CLAUDE.md/threat-model.md/the runbook since early Phase 4 -- the last
major standing gap. Scoping was agreed via AskUserQuestion: a
config-supplied tenant_id + shared-secret token ingest validates
(smaller real implementation, no new PKI), over per-tenant mTLS
certs. This change builds that identity mechanism end to end and
attaches it to every record at the point it enters the system; it
deliberately does NOT build per-tenant write-routing for ClickHouse or
Tantivy -- that's real, separately-scoped follow-up work, disclosed
explicitly everywhere this was previously called undesigned, not
silently left half-done.

New pieces:

- metadata/migrations/0034 + enterprise/internal/rbacstore/
  ingest_credentials.go: a per-tenant bearer credential, only its
  SHA-256 hash ever persisted (same reasoning a password gets hashed,
  not stored raw) -- CreateIngestCredential returns the plaintext
  exactly once, ValidateIngestCredential/RevokeIngestCredential/
  ListIngestCredentialsForTenant round it out.
- enterprise-auth gains -create-ingest-credential-tenant/
  -list-ingest-credentials-tenant/-revoke-ingest-credential (same
  offline-operator-flag shape as every other credential-minting flag in
  this binary) and a new POST /internal/authorize-ingest endpoint
  (internal/authhandler) validating a presented token and resolving its
  tenant -- a genuinely different credential type from session-backed
  /internal/authorize, so it doesn't touch session.Manager at all.
- ingest (AGPL core) gains an optional TenantResolver
  (internal/grpcserver, nil by default) and its HTTP client
  implementation (internal/tenantresolver.HTTPResolver) -- a plain HTTP
  call to enterprise-auth's new endpoint, never an enterprise/ import,
  same "network boundary, not import boundary" shape
  api/authz.HTTPAuthorizer already uses for the query path.
  PushBatch now requires an `authorization: Bearer <token>` gRPC
  metadata entry once a resolver is configured, fails the whole batch
  closed on a missing/invalid credential (never falls back to "no
  tenant"), and attaches the resolved tenant ID to every record as a
  `tenant_id` Kafka message header before producing it.

Verified with real round trips at every layer, no Docker needed:
rbacstore's credential CRUD (skip-gated on live Postgres, same as every
other rbacstore integration test this phase), authhandler's new
endpoint (real HTTP via httptest, including the regression test that a
session token must not validate as an ingest credential), tenantresolver
(real HTTP client against httptest, same pattern as
authz.HTTPAuthorizer's own tests), and grpcserver's PushBatch (fake
resolver/producer -- no resolver leaves messages unchanged, a configured
resolver attaches the right header or fails closed on a bad/missing
token).

Helm: ingest.requireTenantCredential (default false) is a deliberate,
separate opt-in from enterprise.enabled -- turning ENTERPRISE_AUTH_URL
on for ingest requires every agent to already hold a credential or be
refused outright, so it must not default on just because
enterprise.enabled does (same reasoning api.yaml's ENTERPRISE_AUTH_URL
isn't tied to enterprise.enabled directly either). docker-compose.yml
leaves it unset, same as ever.

Docs updated everywhere this was called "undesigned": CLAUDE.md,
docs/architecture.md, docs/security/threat-model.md (including its
summary table, now split into "identity: built" vs "write-routing: not
yet"), docs/phase-4-runbook.md (new §13), enterprise/README.md.

ingest/internal/config/config_test.go

@@ -19,12 +19,16 @@ func TestLoadDefaults(t *testing.T) {
 	if cfg.Batch.FlushIntervalMS != 2000 {
 		t.Errorf("Batch.FlushIntervalMS = %d, want 2000", cfg.Batch.FlushIntervalMS)
 	}
+	if cfg.EnterpriseAuthURL != "" {
+		t.Errorf("EnterpriseAuthURL = %q, want empty (tenant resolution off by default)", cfg.EnterpriseAuthURL)
+	}
 }
 
 func TestLoadOverridesFromEnv(t *testing.T) {
 	t.Setenv("GRPC_LISTEN_ADDR", ":9999")
 	t.Setenv("REDPANDA_BROKERS", "a:9092,b:9092")
 	t.Setenv("CONSUMER_BATCH_MAX_SIZE", "10")
+	t.Setenv("ENTERPRISE_AUTH_URL", "http://enterprise-auth:8082")
 
 	cfg, err := Load()
 	if err != nil {
@@ -39,6 +43,9 @@ func TestLoadOverridesFromEnv(t *testing.T) {
 	if cfg.Batch.MaxSize != 10 {
 		t.Errorf("Batch.MaxSize = %d, want 10", cfg.Batch.MaxSize)
 	}
+	if cfg.EnterpriseAuthURL != "http://enterprise-auth:8082" {
+		t.Errorf("EnterpriseAuthURL = %q, want http://enterprise-auth:8082", cfg.EnterpriseAuthURL)
+	}
 }
 
 func TestLoadInvalidBatchSizeErrors(t *testing.T) {