fix(newznab): make indexer book categories configurable (#1214)

Newznab searches hardcoded category 7000 for ebooks and 3030 for
audiobooks,
so indexers using custom IDs returned no results or the wrong ones. Add
NEWZNAB_EBOOK_CATEGORIES and NEWZNAB_AUDIOBOOK_CATEGORIES (tag lists,
defaulting to 7000 and 3030) and resolve the search categories from
config.

Values are parsed leniently — list or comma/whitespace separated,
non-numeric
entries skipped, duplicates dropped — and fall back to the standard IDs
when
empty, so a cleared field can't silently widen the search to every
category.
NEWZNAB_AUTO_EXPAND remains the way to do that on purpose.

Results carrying a custom ID outside the standard 7000-7999 / 3030
ranges were
typed as "other", which routed custom-category audiobooks as ebooks.
Trust the
searched content type when a result carries a category we explicitly
asked for.

Also drop the unused NEWZNAB_BOOKS / NEWZNAB_AUDIOBOOKS constants from
api.py —
a third copy of the same hardcoding.

Closes #1208

docs/environment-variables.md

@@ -1304,6 +1304,8 @@ Apply per-indexer seed time and ratio preferences from Prowlarr when sending tor
 | `NEWZNAB_ENABLED` | Enable searching for books via a Newznab-compatible indexer | boolean | `false` |
 | `NEWZNAB_URL` | Base URL of your Newznab indexer or aggregator | string | _none_ |
 | `NEWZNAB_API_KEY` | Your Newznab API key (leave blank if not required) | string (secret) | _none_ |
+| `NEWZNAB_EBOOK_CATEGORIES` | Newznab category IDs searched for ebooks. Most indexers use the standard 7000, but some use custom IDs. Leave empty to use 7000. | string (comma-separated) | `7000` |
+| `NEWZNAB_AUDIOBOOK_CATEGORIES` | Newznab category IDs searched for audiobooks. Most indexers use the standard 3030, but some use custom IDs. Leave empty to use 3030. | string (comma-separated) | `3030` |
 | `NEWZNAB_AUTO_EXPAND` | Automatically retry search without category filtering if no results are found | boolean | `false` |
 
 <details>
@@ -1337,6 +1339,24 @@ Your Newznab API key (leave blank if not required)
 - **Type:** string (secret)
 - **Default:** _none_
 
+#### `NEWZNAB_EBOOK_CATEGORIES`
+
+**Ebook Categories**
+
+Newznab category IDs searched for ebooks. Most indexers use the standard 7000, but some use custom IDs. Leave empty to use 7000.
+
+- **Type:** string (comma-separated)
+- **Default:** `7000`
+
+#### `NEWZNAB_AUDIOBOOK_CATEGORIES`
+
+**Audiobook Categories**
+
+Newznab category IDs searched for audiobooks. Most indexers use the standard 3030, but some use custom IDs. Leave empty to use 3030.
+
+- **Type:** string (comma-separated)
+- **Default:** `3030`
+
 #### `NEWZNAB_AUTO_EXPAND`
 
 **Auto-expand search on no results**