Add premiumize cache availability + opt-in OnlyDownloadCached gate

[dabonzo]

What this adds

Two related changes for debrid providers, both scoped to torrents.

1. A real GetAvailableFiles for premiumize. Today the premiumize implementation is a stub:

public Task<IList<DebridClientAvailableFile>> GetAvailableFiles(String hash)
{
    return Task.FromResult<IList<DebridClientAvailableFile>>([]);
}

so premiumize always reports nothing as cached. This implements it against premiumize's
/cache/check, giving it the same meaningful availability answer TorBox already provides.

2. An opt-in OnlyDownloadCached provider setting (default false). When enabled, adding a
torrent whose hash is not cached at the provider is rejected at add time instead of being queued.
For Sonarr/Radarr users that rejection is the point: it lets the *arr fall through to the next
release rather than sitting on a torrent the provider will try to leech from peers.

Why

On premiumize a cached torrent completes instantly and costs a fraction of the fair-use budget, while
an uncached one can stall at 0 B/s from 0 peers indefinitely. Without a working cache check there is
no way to tell those apart before committing to the download.

Design notes

• Off by default — with the setting disabled, behaviour is unchanged. This is additive.
• Provider-general, not premiumize-only. The gate reads
  settings.Current.Provider.OnlyDownloadCached and asks whichever client is configured. On a
  provider with no availability check it logs a warning and stays inactive rather than rejecting
  everything.
• Fails open. A provider error or rate-limit lets the add proceed — if the provider is genuinely
  down the download fails on its own. Failing closed would silently reject a whole library.
• NZB is untouched (torrent.Type != DownloadType.Nzb) — cache checking has no meaning there.
• Uncached hashes are remembered for 30 minutes, keyed per provider, so an *arr re-searching the
  same release each RSS pass doesn't re-query, and an "uncached" verdict can't carry across a
  provider switch.
• The premiumize call deliberately uses raw HTTP rather than the Premiumize.NET library: the library
  never inspects HTTP status codes, so a 429 whose body matches no documented phrase would be
  misread as "not cached". Only a well-formed success answer is allowed to say that.

One caveat worth knowing

Rejected torrents are marked errored, so they are only cleaned up when Delete download when in
error is above 0. The gate logs a warning when that setting is 0, since rejected entries would
otherwise accumulate.

Tests

Adds 18 test methods across three files (PremiumizeDebridClientTest, TorrentsCacheGateTest,
UncachedHashMemoryTest): cache-check parsing, cached/uncached/rate-limited/error responses, the
gate's accept and reject paths with the setting on and off, and the expiry and provider-scoping of
the uncached-hash memory. Full suite green on this branch (243 Service + 37 Web).

Field use

I've been running this for the last 9 days on a Sonarr/Radarr + premiumize setup, as the sole
download client for both protocols. Happy to answer questions about how it behaves in practice.

Overlap with #1014

#1014 (open) touches PremiumizeDebridClient.cs, Torrents.cs and PremiumizeDebridClientTest.cs —
the three files this PR changes most. This branch is rebased on current main (e479404) and applies
cleanly there. If you'd rather merge #1014 first, say so and I'll rebase and resolve.