fix(prowlarr): send categories as repeated query params

[strass]

Symptom

Ebook searches work. Audiobook searches always return HTTP 502, on every
query term and every indexer, with no user-visible setting that differs
between the two:

POST /api/v1/releases/search  {"query":"the hobbit","format":"audiobook",...}
-> 502  {"error":"prowlarr search returned 400 Bad Request"}

Root cause

ProwlarrClient.searchOnce (backend/internal/acquisition/prowlarr.go)
built the category parameter with url.Values.Set and a comma-joined
string from categoriesForFormat:

format	value	encodes to	result
ebook / book	7000	categories=7000	200
audiobook/audio	7000,3030	categories=7000%2C3030	400
"" / any	7000,3030	categories=7000%2C3030	400

Prowlarr's /api/v1/search binds categories as an array of int, which
in ASP.NET means a repeated parameter, not a comma-separated string.
It rejects the comma-joined form during model binding:

"categories": {
  "rawValue": "7000,3030",
  "errors": [{ "errorMessage": "The value '7000,3030' is not valid." }],
  "validationState": "invalid"
}

Librarry surfaces that upstream 400 to the browser as a 502.

Ebook searches were unaffected only incidentally: a single category has
no comma to mis-encode.

Confirmed against Prowlarr directly (same query, same indexers):

categories=7000%2C3030          -> 400
categories=7000&categories=3030 -> 200
categories=7020                 -> 200

Fix

Add categoryListForFormat, which splits the same list so each category
is appended with url.Values.Add, producing repeated parameters.

categoriesForFormat is deliberately retained: fetchIndexerFeed uses
it for the Newznab feed endpoint, where a comma-joined cat parameter
is the correct convention. Only the Prowlarr JSON search path changed.

Verification

• go build ./... — clean
• go test ./... — all pass, no regressions
• New regression test TestProwlarrSearchSendsRepeatedCategoryParamsForAudiobooks
  fails without the fix (categories were comma-joined into one parameter)
  and passes with it
• gofmt clean; patch applies cleanly to pristine upstream at the above revision