Phase 4: real per-tenant ClickHouse isolation via a new enterprise-api binary

Closes the threat model's headline finding for the SQL query path:
enterprise/internal/tenantprovision does real CREATE DATABASE/USER/GRANT
against ClickHouse, and enterprise/internal/chrunner is a per-tenant
connection registry implementing api's SQLRunner interface, resolving
the tenant from the authenticated request identity -- never a
caller-suppliable parameter. Both are wired into a new binary,
enterprise/cmd/enterprise-api, alongside the unchanged single-tenant
api/cmd/api, since AGPL core can never import enterprise/ and Go's own
internal/ package visibility rules meant enterprise/ couldn't implement
core's SQLRunner interface without importing the package that defines
it. That required moving api/internal/{authz,queryapi,dashboards,
querylang/executor,searchclient,httpserver} out of internal/ -- the
minimal set enterprise-api needs to import; querylang's compiler
internals (planner/lexer/parser/ast/ir) and api's own config stay
internal, since nothing outside api needs them directly.

Also finally wires enterprise/internal/audit into queryapi.AuditLogger
(nil since Phase 4 task 4) via a new adapter, and adds live-ClickHouse
integration tests for two of the four adversarial probes named in
docs/phase-4-isolation-design.md's verification plan.

Corrected several overclaims in the docs while writing this up: an
earlier claim that rbacstore's CRUD was "verified against a live
Postgres" was never actually true in this environment (only
internal/audit was, earlier in this phase, before Docker access was
lost) -- threat-model.md, phase-4-runbook.md, CLAUDE.md, and
enterprise/README.md all now distinguish "a real integration test
exists" from "this was confirmed against a live database."

Still not built: Tantivy/free-text tenant isolation
(enterprise/internal/searchclient), and any deployment-topology
mechanism that actually routes traffic to enterprise-api instead of
plain api -- both binaries exist side by side today with nothing
enforcing or flagging which one a deployment runs.

api/httpserver/cors.go

@@ -0,0 +1,24 @@
+// Package httpserver holds cross-handler HTTP concerns for /api. Phase 3
+// introduced a second handler package (dashboards) alongside
+// queryapi, so CORS moved out of individual handlers into one
+// wrap applied around the fully-assembled mux in cmd/api/main.go, rather
+// than each handler package wrapping itself.
+package httpserver
+
+import "net/http"
+
+// WithCORS is deliberately permissive by default (see CORSAllowedOrigin
+// in internal/config) since there's no auth yet and the SvelteKit dev
+// server runs on a different origin. Tighten alongside adding real auth.
+func WithCORS(next http.Handler, allowedOrigin string) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Access-Control-Allow-Origin", allowedOrigin)
+		w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
+		w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
+		if r.Method == http.MethodOptions {
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}