Add source call audio playback to vocabulary suggestions

When the induction loop proposes a new vocabulary term, it now records
which sampled call(s) most likely produced the suggestion. Admins see
a collapsible "▶ source" player under each pending term showing the
audio clip and transcript, so they can hear what was actually said
before approving or dismissing.

- vocabulary_learner: track sampled call docs, attach source_call_ids
  to each pending term via word-overlap search with fallback
- types: VocabularyPendingTerm.source_call_ids?: string[]
- c2api: add getCall(id) using existing GET /calls/{call_id} endpoint
- VocabularyPanel: SourceCallPlayer component — lazy-loads call on
  first expand, shows audio controls + transcript snippet

drb-frontend/lib/c2api.ts

@@ -56,6 +56,7 @@ export const c2api = {
     request(`/nodes/${id}`, { method: "DELETE" }),
 
   // Calls
+  getCall: (callId: string) => request<import("@/lib/types").CallRecord>(`/calls/${callId}`),
   getCalls: (params?: Record<string, string>) => {
     const qs = params ? "?" + new URLSearchParams(params).toString() : "";
     return request<unknown[]>(`/calls${qs}`);