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/querier.go

@@ -22,6 +22,8 @@ type Querier interface {
 	// ============================================
 	AddBookToKoboShelf(ctx context.Context, arg AddBookToKoboShelfParams) (KoboShelves, error)
 	// Library Folders queries
+	// purpose: 'scan' (FS-walker root, default) | 'podcast' (download root;
+	// scanner-skipped, written by the podcast download worker instead).
 	AddLibraryFolder(ctx context.Context, arg AddLibraryFolderParams) (LibraryFolders, error)
 	// Second consecutive missing scan: hide the item from all browsing while
 	// preserving reading history in case the file returns.