feat(telegram): candidate list/delete/clear commands

私聊新增 /candidate [页码]（待决策列表）、/candi_del（标题按钮逐条删除，删后原地重建键盘、可翻页连续删）、/candi_clear（全部/待决策/取消）。重构 handle_callback 为前缀分派（cdel/cpage/cclr，保留 add/ign/blk/fav）；repository 加 list_pending_candidates/delete_all_candidates；LibraryService 加 pending_candidates/delete_candidate/clear_candidates 并刷看板缓存。

reaction_board/repository.py

@@ -919,6 +919,16 @@ class Repository:
             return [dict(row) for row in rows]
         return self.db.read(run)
 
+    def list_pending_candidates(self) -> list[dict[str, Any]]:
+        """待决策候选（``decision IS NULL``），按分数倒序。"""
+        def run(conn: sqlite3.Connection) -> list[dict[str, Any]]:
+            rows = conn.execute(
+                "SELECT * FROM candidate WHERE decision IS NULL "
+                "ORDER BY COALESCE(score, 0) DESC, first_seen DESC, aid"
+            ).fetchall()
+            return [dict(row) for row in rows]
+        return self.db.read(run)
+
     def mark_candidate_decision(
         self, aid: int, decision: str, *, now: int | None = None
     ) -> None:
@@ -944,6 +954,16 @@ class Repository:
             "DELETE FROM candidate WHERE aid = ?", (int(aid),)
         ))
 
+    def delete_all_candidates(self, *, pending_only: bool = False) -> int:
+        """删除候选：``pending_only=True`` 只删待决策，否则清空整表；返回删除数。"""
+        def run(conn: sqlite3.Connection) -> int:
+            if pending_only:
+                cur = conn.execute("DELETE FROM candidate WHERE decision IS NULL")
+            else:
+                cur = conn.execute("DELETE FROM candidate")
+            return max(cur.rowcount, 0)
+        return self.db.write(run)
+
     def delete_expired_candidates(
         self, *, ttl_days: int, now: int | None = None
     ) -> int: