Admin Replay: re-run the pipeline over past calls in a sandbox (#170)

Correlation has only ever been measured through live AI windows: days of
wall time per change, and the 09-20→22 window was invalidated outright by
unfunded AI accounts (#169). Recordings are kept regardless of AI, so the
traffic to measure against already exists.

- internal/replay.py: runs a time range of real calls through the live
  pipeline code in original order, clock pinned per call, into
  replay_runs/{run_id}/calls|incidents. Modes: audio (re-transcribe),
  transcripts (re-extract), reuse (correlation only from a prior run's
  scenes). Simulates the idle-resolve and orphan-recorrelation sweeps on
  virtual time. No alerts, summaries, vocab, AI-health alerts or pending
  terms. One run at a time, <=5000 calls, <=7 days.
- firestore.py: ContextVar sandbox redirect for calls/incidents.
- clock.py: ContextVar-pinnable now(), used on the correlation path.
- feature_flags.py: ContextVar flag override so replay runs with live AI off.
- upload.py: scene loop extracted to _extract_and_correlate, shared by the
  live pipeline and replay so replay measures the code that runs live.
- resolved_via on every incident resolve, so a real clear can be told
  from the idle timeout — live and in replay.
- routers/replay.py + /admin Replay tab: estimate, start, compare runs,
  drill into incidents with audio.

Reviewed by drb-correlation-review; its leak and fidelity findings are
fixed and covered by tests. c2-core: 456 pass. Frontend typecheck not run
(no Node on the authoring box).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/app/admin/page.tsx

@@ -2,6 +2,7 @@
 
 import { useAuth } from "@/components/AuthProvider";
 import { c2api } from "@/lib/c2api";
+import { ReplayTab } from "@/components/admin/ReplayTab";
 import { useEffect, useState, useRef, useCallback } from "react";
 import { useRouter } from "next/navigation";
 import type { UserRecord, AuditEntry, UserRole, CallRecord } from "@/lib/types";
@@ -1227,11 +1228,12 @@ function SttEvalTab() {
 // Main admin page
 // ---------------------------------------------------------------------------
 
-type AdminTab = "features" | "correlation" | "users" | "audit" | "calls" | "eval";
+type AdminTab = "features" | "correlation" | "replay" | "users" | "audit" | "calls" | "eval";
 
 const TAB_LABELS: { key: AdminTab; label: string }[] = [
   { key: "features",    label: "AI Features" },
   { key: "correlation", label: "Correlation Debug" },
+  { key: "replay",      label: "Replay" },
   { key: "calls",       label: "Calls" },
   { key: "eval",        label: "STT Eval" },
   { key: "users",       label: "Users" },
@@ -1256,7 +1258,7 @@ export default function AdminPage() {
   if (!isAdmin) return null;
 
   // Users/Audit tabs benefit from full width; everything else is narrow
-  const wide = tab === "users" || tab === "audit";
+  const wide = tab === "users" || tab === "audit" || tab === "replay";
 
   return (
     <div className={`space-y-6 ${wide ? "" : "max-w-2xl"}`}>
@@ -1278,6 +1280,7 @@ export default function AdminPage() {
 
       {tab === "features"    && <FeaturesTab />}
       {tab === "correlation" && <CorrelationDebugTab />}
+      {tab === "replay"      && <ReplayTab />}
       {tab === "calls"       && <StaleCallsTab />}
       {tab === "eval"        && <SttEvalTab />}
       {tab === "users"       && <UsersTab currentUid={user?.uid ?? ""} />}