fix(import): wire ImportFileStore to bypass Asynq type assertion; add pb-init collections

- Add ImportFileStore interface to bookstore package
- Add ImportFileStore field to backend.Dependencies
- Wire ImportFileStore: store in cmd/backend/main.go
- handlers_import.go: use s.deps.ImportFileStore.PutImportFile instead of
  broken s.deps.Producer.(*storage.Store) type assertion (fails when Asynq active)
- pb-init-v3.sh: add import_tasks and notifications collection definitions

scripts/pb-init-v3.sh

@@ -299,6 +299,39 @@ create "translation_jobs" '{
     {"name":"heartbeat_at", "type":"date"}
   ]}'
 
+create "import_tasks" '{
+  "name":"import_tasks","type":"base","fields":[
+    {"name":"slug",              "type":"text",  "required":true},
+    {"name":"title",             "type":"text",  "required":true},
+    {"name":"file_name",         "type":"text"},
+    {"name":"file_type",         "type":"text"},
+    {"name":"object_key",        "type":"text"},
+    {"name":"author",            "type":"text"},
+    {"name":"cover_url",         "type":"text"},
+    {"name":"genres",            "type":"text"},
+    {"name":"summary",           "type":"text"},
+    {"name":"book_status",       "type":"text"},
+    {"name":"worker_id",         "type":"text"},
+    {"name":"initiator_user_id", "type":"text"},
+    {"name":"status",            "type":"text",  "required":true},
+    {"name":"chapters_done",     "type":"number"},
+    {"name":"chapters_total",    "type":"number"},
+    {"name":"error_message",     "type":"text"},
+    {"name":"started",           "type":"date"},
+    {"name":"finished",          "type":"date"},
+    {"name":"heartbeat_at",      "type":"date"}
+  ]}'
+
+create "notifications" '{
+  "name":"notifications","type":"base","fields":[
+    {"name":"user_id", "type":"text","required":true},
+    {"name":"title",   "type":"text","required":true},
+    {"name":"message", "type":"text"},
+    {"name":"link",    "type":"text"},
+    {"name":"read",    "type":"bool"},
+    {"name":"created", "type":"date"}
+  ]}'
+
 create "ai_jobs" '{
   "name":"ai_jobs","type":"base","fields":[
     {"name":"kind",          "type":"text",  "required":true},