Exam bank: corpus coverage dashboard (/exam-marker/corpus)

New "Exam bank" page displaying the state of the collected past-paper corpus:
per-specification coverage of question papers / mark schemes / examiner reports,
with global + filtered rollups, board/level/subject filters, a "sciences only"
default, and an expandable per-session table (QP/MS/ER present ✓/–). Reads
GET /api/exam/corpus. Dashboard gets an "Exam bank" entry point.

examRepository.getCorpus + corpus types. tsc clean on the changed files.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/types/exam.types.ts

@@ -371,3 +371,28 @@ export interface CreateCustomPaperResult {
   n_questions: number;
   n_response_areas: number;
 }
+
+// ── Exam-bank corpus coverage (state of the collected bank) ───────────────────
+export type CorpusDocType = 'QP' | 'MS' | 'ER';
+export interface CorpusPaper {
+  paper_code: string | null;
+  session: string | null;
+  tier: string | null;
+  docs: Partial<Record<CorpusDocType, boolean>>;
+  exam_codes: Partial<Record<CorpusDocType, string>>;
+}
+export interface CorpusSpec {
+  spec_code: string;
+  subject: string;
+  level: string;
+  board: string;
+  first_teach: string | null;
+  n_papers: number;
+  counts: Record<CorpusDocType, number>;
+  papers: CorpusPaper[];
+}
+export interface CorpusBoard { board: string; n_specs: number; specs: CorpusSpec[] }
+export interface CorpusResponse {
+  totals: { specs: number; papers: number; sessions: number; QP: number; MS: number; ER: number };
+  boards: CorpusBoard[];
+}