feat: add PDF/EPUB import functionality

- Add ImportTask/ImportResult types to domain.go
- Add TypeImportBook to asynqqueue for task routing
- Add CreateImportTask to producer and storage layers
- Add ClaimNextImportTask/FinishImportTask to Consumer interfaces
- Add import task handling to runner (polling + Asynq handler)
- Add BookImporter interface to bookstore for PDF/EPUB parsing
- Add backend API endpoints: POST/GET /api/admin/import
- Add SvelteKit UI at /admin/import with task list
- Add nav link in admin layout

Note: PDF/EPUB parsing is a placeholder - needs external library integration.

backend/internal/asynqqueue/tasks.go

@@ -23,6 +23,7 @@ const (
 	TypeAudioGenerate   = "audio:generate"
 	TypeScrapeBook      = "scrape:book"
 	TypeScrapeCatalogue = "scrape:catalogue"
+	TypeImportBook      = "import:book"
 )
 
 // AudioPayload is the Asynq job payload for audio generation tasks.
@@ -44,3 +45,12 @@ type ScrapePayload struct {
 	FromChapter int    `json:"from_chapter"` // 0 unless Kind=="book_range"
 	ToChapter   int    `json:"to_chapter"`   // 0 unless Kind=="book_range"
 }
+
+// ImportPayload is the Asynq job payload for PDF/EPUB import tasks.
+type ImportPayload struct {
+	PBTaskID  string `json:"pb_task_id"`
+	Slug      string `json:"slug"`
+	Title     string `json:"title"`
+	FileType  string `json:"file_type"` // "pdf" or "epub"
+	ObjectKey string `json:"object_key"` // MinIO path to uploaded file
+}