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.sql.go

@@ -123,17 +123,20 @@ func (q *Queries) AddBookToKoboShelf(ctx context.Context, arg AddBookToKoboShelf
 }
 
 const AddLibraryFolder = `-- name: AddLibraryFolder :one
-INSERT INTO library_folders (library_id, folder_path) VALUES ($1, $2) RETURNING id, library_id, folder_path, created_at, purpose
+INSERT INTO library_folders (library_id, folder_path, purpose) VALUES ($1, $2, $3) RETURNING id, library_id, folder_path, created_at, purpose
 `
 
 type AddLibraryFolderParams struct {
 	LibraryID  pgtype.UUID `db:"library_id" json:"library_id"`
 	FolderPath string      `db:"folder_path" json:"folder_path"`
+	Purpose    string      `db:"purpose" json:"purpose"`
 }
 
 // Library Folders queries
+// purpose: 'scan' (FS-walker root, default) | 'podcast' (download root;
+// scanner-skipped, written by the podcast download worker instead).
 func (q *Queries) AddLibraryFolder(ctx context.Context, arg AddLibraryFolderParams) (LibraryFolders, error) {
-	row := q.db.QueryRow(ctx, AddLibraryFolder, arg.LibraryID, arg.FolderPath)
+	row := q.db.QueryRow(ctx, AddLibraryFolder, arg.LibraryID, arg.FolderPath, arg.Purpose)
 	var i LibraryFolders
 	err := row.Scan(
 		&i.ID,