feat(settings): storage budgets and retention in settings UI, tmdbMaxSize default 5000 (INT-3)

- Automation section gets storage budget, retention and cache TTL cards bound to settings.storage
- Metadata section drops deprecated cache.tmdbTtlHours / cache.emptyResultTtlSec / identity.poolArchiveDays inputs
- cache.tmdbMaxSize default 256 -> 5000; migration 0051 bumps stored values still at the old default

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/runbook/storage.md

@@ -34,7 +34,7 @@ PATCH /api/v1/settings {"storage":{"retentionDays":{"observation":60}}}
 被取代的旧字段(保留在 schema 里兼容已存 JSON,服务端不再读取,已列入 `DEPRECATED_SETTINGS_FIELDS`):
 `cache.tmdbTtlHours` → `cacheTtl.tmdbSearchDays / tmdbDetailDays`,`cache.emptyResultTtlSec` → `cacheTtl.tmdbMissDays`,
 `identity.poolArchiveDays` → `retentionDays.identityPoolArchive`。设置页「元数据」里这三项接 UI 时要换成新字段。
-`cache.tmdbMaxSize`(条数上限,缺省 256)仍然有效。TTL 拉长后条数上限会先起作用,需要时调大。
+`cache.tmdbMaxSize`(条数上限,缺省 5000)仍然有效。TTL 拉长后条数上限会先起作用,需要时调大。
 
 ## 缓存有效期