feat(cleanup): 解耦 request_candidates 与 provider_api_keys 生命周期

- 移除 request_candidates.key_id 对 provider_api_keys 的外键约束(含迁移脚本)
- 删除 Key 时不再级联删除候选记录，改为独立按保留天数定时清理
- 新增 request_candidates_retention_days / request_candidates_cleanup_batch_size 配置项
- batch_delete_task 增加 lock_timeout 及超时自动降批重试机制
- cleanup_key_references 提取阶段化清理流程，移除 RequestCandidate 联动删除
- 前端 CleanupPolicySection 新增候选记录保留天数和清理批次配置

Closes #227

Co-authored-by: Entropy-Xu <entropy.xu@cloudhabitatsh.com>

alembic/versions/20260312_1915_b7c8d9e0f1a2_decouple_request_candidates_key_id_fk.py

@@ -0,0 +1,64 @@
+"""decouple request_candidates.key_id foreign key from provider_api_keys lifecycle
+
+Revision ID: b7c8d9e0f1a2
+Revises: c1d2e3f4a5b6
+Create Date: 2026-03-12 19:15:00.000000+00:00
+
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "b7c8d9e0f1a2"
+down_revision = "c1d2e3f4a5b6"
+branch_labels = None
+depends_on = None
+
+
+def _fk_exists(constraint_name: str, table_name: str) -> bool:
+    bind = op.get_bind()
+    result = bind.execute(
+        sa.text(
+            "SELECT 1 FROM pg_constraint c "
+            "JOIN pg_class r ON c.conrelid = r.oid "
+            "JOIN pg_namespace n ON r.relnamespace = n.oid "
+            "WHERE c.conname = :name AND r.relname = :table "
+            "  AND n.nspname = current_schema() AND c.contype = 'f'"
+        ),
+        {"name": constraint_name, "table": table_name},
+    )
+    return result.scalar() is not None
+
+
+def upgrade() -> None:
+    if _fk_exists("request_candidates_key_id_fkey", "request_candidates"):
+        op.drop_constraint(
+            "request_candidates_key_id_fkey", "request_candidates", type_="foreignkey"
+        )
+
+
+def downgrade() -> None:
+    bind = op.get_bind()
+    bind.execute(
+        sa.text(
+            "UPDATE request_candidates rc "
+            "SET key_id = NULL "
+            "WHERE key_id IS NOT NULL "
+            "  AND NOT EXISTS ("
+            "    SELECT 1 FROM provider_api_keys pak WHERE pak.id = rc.key_id"
+            "  )"
+        )
+    )
+    if not _fk_exists("request_candidates_key_id_fkey", "request_candidates"):
+        op.create_foreign_key(
+            "request_candidates_key_id_fkey",
+            "request_candidates",
+            "provider_api_keys",
+            ["key_id"],
+            ["id"],
+            ondelete="CASCADE",
+        )