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
- 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/taskqueue/taskqueue.go

@@ -33,6 +33,10 @@ type Producer interface {
 	// returns the assigned PocketBase record ID.
 	CreateTranslationTask(ctx context.Context, slug string, chapter int, lang string) (string, error)
 
+	// CreateImportTask inserts a new import task with status=pending and
+	// returns the assigned PocketBase record ID.
+	CreateImportTask(ctx context.Context, slug, title, fileType, objectKey string) (string, error)
+
 	// CancelTask transitions a pending task to status=cancelled.
 	// Returns ErrNotFound if the task does not exist.
 	CancelTask(ctx context.Context, id string) error
@@ -59,6 +63,11 @@ type Consumer interface {
 	// Returns (zero, false, nil) when the queue is empty.
 	ClaimNextTranslationTask(ctx context.Context, workerID string) (domain.TranslationTask, bool, error)
 
+	// ClaimNextImportTask atomically finds the oldest pending import task,
+	// sets its status=running and worker_id=workerID, and returns it.
+	// Returns (zero, false, nil) when the queue is empty.
+	ClaimNextImportTask(ctx context.Context, workerID string) (domain.ImportTask, bool, error)
+
 	// FinishScrapeTask marks a running scrape task as done and records the result.
 	FinishScrapeTask(ctx context.Context, id string, result domain.ScrapeResult) error
 
@@ -68,6 +77,9 @@ type Consumer interface {
 	// FinishTranslationTask marks a running translation task as done and records the result.
 	FinishTranslationTask(ctx context.Context, id string, result domain.TranslationResult) error
 
+	// FinishImportTask marks a running import task as done and records the result.
+	FinishImportTask(ctx context.Context, id string, result domain.ImportResult) error
+
 	// FailTask marks a task (scrape, audio, or translation) as failed with an error message.
 	FailTask(ctx context.Context, id, errMsg string) error
 
@@ -104,4 +116,11 @@ type Reader interface {
 	// GetTranslationTask returns the most recent translation task for cacheKey.
 	// Returns (zero, false, nil) if not found.
 	GetTranslationTask(ctx context.Context, cacheKey string) (domain.TranslationTask, bool, error)
+
+	// ListImportTasks returns all import tasks sorted by started descending.
+	ListImportTasks(ctx context.Context) ([]domain.ImportTask, error)
+
+	// GetImportTask returns a single import task by ID.
+	// Returns (zero, false, nil) if not found.
+	GetImportTask(ctx context.Context, id string) (domain.ImportTask, bool, error)
 }