feat(podcasts): scanner exclusion — podcasts libraries are feed-managed (task 1.2)

WHY: podcast download roots hold fetcher-written episode files; the FS
walker must never discover them (invariant 4) — a walk would double-
ingest episodes as scanner items and fight the fetcher's sole-writer
rule (D9). Every scan/watch path funnels through MediaScanner.SetFolders
(StartScanner, ScanLibrary jobs incl. scoped single-folder scans,
watch mode), so the filter lives there once: folders resolving to a
podcasts-type library are dropped before registration, with a log line.

Belt and braces: ScanLibrary's per-folder scope refuses purpose='podcast'
folders explicitly (400) — the panel-level path that bypasses type
resolution. AddLibraryFolder now derives purpose from the library type
('podcast' for podcasts libraries, enforced single download root;
'scan' default elsewhere), so folder purpose and library type cannot
diverge through the API.

internal/database/queries/queries.sql

@@ -97,7 +97,9 @@ DELETE FROM libraries WHERE id = $1;
 
 -- Library Folders queries
 -- name: AddLibraryFolder :one
-INSERT INTO library_folders (library_id, folder_path) VALUES ($1, $2) RETURNING *;
+-- purpose: 'scan' (FS-walker root, default) | 'podcast' (download root;
+-- scanner-skipped, written by the podcast download worker instead).
+INSERT INTO library_folders (library_id, folder_path, purpose) VALUES ($1, $2, $3) RETURNING *;
 
 -- name: GetLibraryFolders :many
 SELECT * FROM library_folders WHERE library_id = $1 ORDER BY created_at;