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-c2-core/app/main.py

@@ -17,7 +17,7 @@ from app.internal.auth import (
     require_node_service_or_firebase_token,
 )
 from app.routers import nodes, systems, calls, upload, tokens, incidents, alerts, admin, trips, places, links, users
-from app.routers import enrollment, media, org, waitlist, telemetry
+from app.routers import enrollment, media, org, waitlist, telemetry, replay
 from app.internal import dynsec
 from app.internal import firestore as fstore
 
@@ -129,6 +129,7 @@ app.include_router(trips.router,     dependencies=[Depends(require_service_or_fi
 app.include_router(places.router,    dependencies=[Depends(require_service_or_firebase_token)])
 app.include_router(upload.router)    # auth is per-node, handled inline
 app.include_router(admin.router)     # auth is per-endpoint (read: firebase, write: admin)
+app.include_router(replay.router)    # auth: admin only (every route spends or reads a replay run)
 app.include_router(users.router)     # auth: admin only
 app.include_router(links.router)     # auth is per-endpoint (generate: firebase, resolve: service key)
 app.include_router(enrollment.router)  # public; auth is the enrollment/pickup-secret tokens, checked inline