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/internal/area_context.py

@@ -448,6 +448,8 @@ async def add_pending(system_id: str, talkgroup_id: Any, entries: list[dict]) ->
     """
     from app.internal import firestore as fstore
 
+    if fstore.in_sandbox():
+        return 0  # a replay proposes nothing to the live review queue
     if not system_id or talkgroup_id is None or not entries:
         return 0
     system_doc = await fstore.doc_get("systems", system_id)