fix(question_gen): rename finished_at to ended_at per schema spec

Aligns DDL column name with research-wiki/schemas/question-gen-runs.md
which specifies 'ended_at' (not 'finished_at').

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/question_gen/run_store.py

@@ -97,7 +97,7 @@ CREATE TABLE IF NOT EXISTS question_gen_runs (
     git_sha         TEXT NOT NULL,
     config_snapshot TEXT,
     started_at      TEXT NOT NULL,
-    finished_at     TEXT,
+    ended_at        TEXT,
     status          TEXT NOT NULL DEFAULT 'running',
     total_slots     INTEGER,
     accepted        INTEGER,
@@ -215,7 +215,7 @@ class QuestionGenStore:
         self._conn.execute(
             """
             UPDATE question_gen_runs
-            SET finished_at=?, status=?, total_slots=?, accepted=?, rejected=?, heavy_sampled=?
+            SET ended_at=?, status=?, total_slots=?, accepted=?, rejected=?, heavy_sampled=?
             WHERE run_id=?
             """,
             (