custom_folders: add category / not_category filter primitives

[jakeyr]

Summary

Adds two filter primitives — category and not_category — to the Virtual Folders feature, letting a folder scope to torrents added by a specific *arr without any out-of-band metadata.

The qBit category set by the *arr at torrent-add time is already a hot field on IndexEntry (populated in pkg/server/qbit/http.go). This change just exposes it to the existing virtual-folder filter chain.

Why

A common ask is "show me only the torrents Sonarr added" or "everything Radarr added except 4K" as a virtual folder. The metadata is already there; the filter language just couldn't reference it.

With this PR, configs like the following work:

"custom_folders": {
  "sonarr":   { "filters": { "category": "tv-sonarr" } },
  "radarr":   { "filters": { "category": "radarr" } },
  "whisparr": { "filters": { "category": "backup" } },
  "non-radarr": { "filters": { "not_category": "radarr" } }
}

category interacts cleanly with the existing AND-stack — e.g. a "Sonarr + 2160p" folder is just { "category": "tv-sonarr", "include": "2160p" }.

What changed

• pkg/manager/custom_folders.go — two new filter constants (filterByCategory, filterByNotCategory); matchesFilter and checkSingleFilter take a new category parameter; two case branches do exact-match / negated-match against the torrent's Category.
• pkg/storage/entry.go — surface Category on EntryMetaInfo so the metadata-only iterator (ForEachMeta) can pass it without a disk read. The field is already on IndexEntry, so no new state.
• pkg/manager/entry.go — the single call site in getCustomFolderChildren passes meta.Category.
• pkg/manager/custom_folders_test.go — table tests for: simple match / mismatch, empty-category rejection (an empty Category never matches a non-empty filter), not_category positive + negative, and category in an AND-stack with include (Sonarr+2160p match, Sonarr+1080p reject, Radarr+2160p reject, empty+2160p reject).

Empty-category behavior is intentional and tested: a torrent with no category never matches a category: "X" filter ("" == "tv-sonarr" is false), so older torrents that predate qBit category tracking simply don't show up in category-filtered folders — they remain visible under __all__, the per-provider folder, and any non-category virtual folder.

Test plan

• go test ./pkg/manager/... -run TestMatchesFilterCategory — all 9 new cases pass
• go test ./... — full suite green
• Built locally and validated end-to-end against a live cache:
  • sonarr folder (filter category: tv-sonarr) returned 394 torrents
  • radarr folder (filter category: radarr) returned 69 torrents
  • Spot-checked entries against torrents.json to confirm the category labels match

Notes for the reviewer

• No config-schema change beyond accepting two new filter-type strings; existing virtual-folder configs continue to work unchanged.
• The new category parameter is threaded through matchesFilter / checkSingleFilter — there is exactly one call site (the metadata-only iterator in getCustomFolderChildren), and the signature change is local to the manager package.
• I'm separately running a small one-shot backfill on my deploy that fills in IndexEntry.Category for entries created before this field was tracked, by reading qBit's torrents.json. That's not part of this PR — happy to send it as a follow-up if it's useful upstream.

[sirrobot01]

Nice work @jakeyr

But this needs to be added to the UI as well.

[kebbob]

Ah! I could see "Filter key (e.g. name, category)" is listed in an empty input in the Virtual Folder filter, but I couldn't get it to work. Thought I was doing it wrong.

+1 for this being added