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.

enterprise/internal/authhandler/authhandler.go

@@ -7,9 +7,16 @@
 // signed tokens with a different Role claim, so one validation path
 // handles both, and the Role claim (not which header carried it) is what
 // determines whether the result looks like a human or a service identity.
+//
+// POST /internal/authorize-ingest is a sibling endpoint, same network-
+// boundary shape but for a different caller (`ingest`, core/AGPL, not
+// api/authz) and a different credential type (an ingest bearer token
+// checked against rbacstore's ingest_credentials table, not a
+// session.Manager JWT) -- see ingestCredentialValidator's doc comment.
 package authhandler
 
 import (
+	"context"
 	"encoding/json"
 	"log/slog"
 	"net/http"
@@ -32,19 +39,32 @@ type Features struct {
 	SAMLEnabled bool
 }
 
-type Handler struct {
-	logger   *slog.Logger
-	manager  *session.Manager
-	features Features
+// ingestCredentialValidator is the narrow interface POST
+// /internal/authorize-ingest needs -- *rbacstore.Store is the production
+// implementation. Unlike session-backed /internal/authorize, this
+// endpoint validates a completely different credential type (an ingest
+// bearer token, checked against enterprise/internal/rbacstore's
+// ingest_credentials table, never a session.Manager-signed JWT), so it
+// needs a dependency session.Manager alone can't supply.
+type ingestCredentialValidator interface {
+	ValidateIngestCredential(ctx context.Context, token string) (tenantID string, err error)
 }
 
-func New(logger *slog.Logger, manager *session.Manager, features Features) *Handler {
-	return &Handler{logger: logger, manager: manager, features: features}
+type Handler struct {
+	logger            *slog.Logger
+	manager           *session.Manager
+	features          Features
+	ingestCredentials ingestCredentialValidator
+}
+
+func New(logger *slog.Logger, manager *session.Manager, features Features, ingestCredentials ingestCredentialValidator) *Handler {
+	return &Handler{logger: logger, manager: manager, features: features, ingestCredentials: ingestCredentials}
 }
 
 func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("POST /internal/authorize", h.handleAuthorize)
 	mux.HandleFunc("GET /auth/features", h.handleFeatures)
+	mux.HandleFunc("POST /internal/authorize-ingest", h.handleAuthorizeIngest)
 }
 
 type featuresResponse struct {
@@ -100,6 +120,32 @@ func (h *Handler) handleAuthorize(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+type authorizeIngestResponse struct {
+	TenantID string `json:"tenant_id"`
+}
+
+// handleAuthorizeIngest is ingest/internal/grpcserver.HTTPTenantResolver's
+// server side -- ingest calls this once per PushBatch (with the bearer
+// token the agent presented) to resolve which tenant the batch belongs
+// to, the network-boundary equivalent of api/authz.HTTPAuthorizer
+// calling /internal/authorize, for a different credential type.
+func (h *Handler) handleAuthorizeIngest(w http.ResponseWriter, r *http.Request) {
+	token := bearerToken(r.Header.Get("Authorization"))
+	if token == "" {
+		http.Error(w, "no credentials presented", http.StatusUnauthorized)
+		return
+	}
+
+	tenantID, err := h.ingestCredentials.ValidateIngestCredential(r.Context(), token)
+	if err != nil {
+		http.Error(w, "invalid ingest credential", http.StatusUnauthorized)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(authorizeIngestResponse{TenantID: tenantID})
+}
+
 func bearerToken(header string) string {
 	const prefix = "Bearer "
 	if !strings.HasPrefix(header, prefix) {