Add Transmission/NZBGet and per-client paths and much more

Extend multi-download-client support to include Transmission and NZBGet and introduce per-client custom download paths. Adds protocol mapping and new client types, Transmission/NZBGet integration services, API CRUD and validation changes, UI components/modal updates and live path previews, and manager routing by protocol. Includes DB migrations (download_path on download_history, interactive_search_access on users), schema updates, and related processor/service fixes and tests to ensure backward compatibility and proper path resolution.

src/lib/constants/audio-formats.ts

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+/**
+ * Component: Audio Format Constants
+ * Documentation: documentation/phase3/file-organization.md
+ *
+ * Centralized audio format definitions used across the application.
+ * Add new formats here to enable support in all subsystems.
+ */
+
+/**
+ * All supported audio file extensions for audiobook detection and file organization.
+ * Used by: file-organizer.ts, files-hash.ts
+ */
+export const AUDIO_EXTENSIONS = [
+  '.m4b',
+  '.m4a',
+  '.mp3',
+  '.mp4',
+  '.aa',
+  '.aax',
+  '.flac',
+  '.ogg',
+] as const;
+
+/**
+ * Audio formats supported by the chapter merger (FFmpeg concat + M4B output).
+ * Formats here can be detected, probed, ordered, and merged into a single M4B.
+ * Note: .aa/.aax excluded (DRM-protected, cannot be decoded by FFmpeg without keys).
+ * Note: .ogg excluded (FFmpeg concat demuxer does not support Ogg container).
+ */
+export const CHAPTER_MERGE_FORMATS = [
+  '.mp3',
+  '.m4a',
+  '.m4b',
+  '.mp4',
+  '.aac',
+  '.flac',
+] as const;
+
+/**
+ * Audio formats supported by metadata tagging via FFmpeg.
+ * Each format maps to a specific FFmpeg output format flag and tagging strategy.
+ */
+export const METADATA_TAG_FORMATS = [
+  '.m4b',
+  '.m4a',
+  '.mp3',
+  '.mp4',
+  '.flac',
+] as const;
+
+/**
+ * Formats that use MP4/M4A container tags (iTunes-style metadata).
+ * These use `-f mp4` output format in FFmpeg.
+ */
+export const MP4_CONTAINER_FORMATS = ['.m4b', '.m4a', '.mp4'] as const;
+
+/**
+ * Audio format identifiers detectable in torrent/NZB titles.
+ * Used by Prowlarr service for metadata extraction and ranking algorithm for scoring.
+ */
+export const TORRENT_TITLE_FORMATS = ['M4B', 'M4A', 'MP3', 'FLAC'] as const;
+
+export type TorrentTitleFormat = (typeof TORRENT_TITLE_FORMATS)[number];
+
+/**
+ * Type helper for the format field on TorrentResult.
+ * 'OTHER' is used when no recognized format is detected in the title.
+ */
+export type AudioFormat = TorrentTitleFormat | 'OTHER';