admin: STT eval harness — record human-verified transcripts, measure real WER (#163)

Backend: three new routes on the calls router, deliberately separate from
PATCH /{call_id}/transcript (a production correction with real side effects
-- re-extraction, incident unlinking, vocabulary learning). This is pure
measurement and must never share that code path.

  GET  /calls/eval-queue        -- calls with a transcript but no
                                    eval_transcript yet, paged (same bounded-
                                    window-plus-cursor shape as /search)
  PUT  /{call_id}/eval-transcript -- records eval_transcript/_by/_at only;
                                      never touches transcript/transcript_corrected
  GET  /calls/eval-stats        -- eval_count + average word error rate of
                                    the raw and corrected machine transcripts
                                    against the human-verified ones

internal/wer.py: standard word-level Levenshtein WER. Returns None (not 0.0)
when the reference is empty -- a call nobody transcribed must not score as a
perfect match.

Frontend: a new "STT Eval" tab on /admin -- one call at a time, audio player,
a textarea pre-filled with the machine transcript to correct into ground
truth, Save & next / Skip, running WER stats at the top. Built for working a
handful of calls at a time over however many sittings it takes, not a
one-shot form: the queue auto-refills from where the last save left off.

Verified: 438 pass, 0 fail (12 new backend tests). Frontend is UNVERIFIED --
this box has no Node.js/npm (confirmed absent), so neither typecheck nor the
dev server could be run. Matches existing code patterns and the CallRecord/
c2api types by manual review only.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

drb-frontend/lib/c2api.ts

@@ -100,6 +100,30 @@ export const c2api = {
   closeStallCalls: (olderThanMinutes: number, dryRun: boolean) =>
     request<{ dry_run: boolean; older_than_minutes: number; count: number; call_ids: string[] }>(`/calls/close-stale?older_than_minutes=${olderThanMinutes}&dry_run=${dryRun}`, { method: "POST" }),
 
+  // STT eval harness (server-26#163) — separate from patchTranscript above,
+  // which is a production correction with real side effects (re-extraction,
+  // incident unlinking, vocabulary learning). This is pure measurement.
+  getEvalQueue: (limit: number, cursor?: string | null) => {
+    const qs = new URLSearchParams({ limit: String(limit) });
+    if (cursor) qs.set("cursor", cursor);
+    return request<{
+      calls: import("@/lib/types").CallRecord[];
+      next_cursor: string | null;
+      scanned: number;
+      matched: number;
+      window_exhausted: boolean;
+    }>(`/calls/eval-queue?${qs.toString()}`);
+  },
+  getEvalStats: () =>
+    request<{ eval_count: number; raw_wer: number | null; corrected_wer: number | null }>(
+      "/calls/eval-stats"
+    ),
+  putEvalTranscript: (callId: string, text: string) =>
+    request<{ ok: boolean; call_id: string }>(`/calls/${callId}/eval-transcript`, {
+      method: "PUT",
+      body: JSON.stringify({ text }),
+    }),
+
   // Incidents
   getIncidents: (params?: { status?: string; type?: string }) => {
     const qs = params ? "?" + new URLSearchParams(params as Record<string, string>).toString() : "";