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/domain/domain.go

@@ -170,6 +170,29 @@ type TranslationResult struct {
 	ErrorMessage string `json:"error_message,omitempty"`
 }
 
+// ImportTask represents a PDF/EPUB import job stored in PocketBase.
+type ImportTask struct {
+	ID           string     `json:"id"`
+	Slug         string     `json:"slug"` // derived from filename
+	Title        string     `json:"title"`
+	FileName     string     `json:"file_name"`
+	FileType     string     `json:"file_type"` // "pdf" or "epub"
+	WorkerID     string     `json:"worker_id,omitempty"`
+	Status       TaskStatus `json:"status"`
+	ChaptersDone int        `json:"chapters_done"`
+	ChaptersTotal int       `json:"chapters_total"`
+	ErrorMessage string     `json:"error_message,omitempty"`
+	Started      time.Time  `json:"started"`
+	Finished     time.Time  `json:"finished,omitempty"`
+}
+
+// ImportResult is the outcome reported by the runner after finishing an ImportTask.
+type ImportResult struct {
+	Slug             string `json:"slug,omitempty"`
+	ChaptersImported int    `json:"chapters_imported"`
+	ErrorMessage     string `json:"error_message,omitempty"`
+}
+
 // AIJob represents an AI generation task tracked in PocketBase (ai_jobs collection).
 type AIJob struct {
 	ID string `json:"id"`