feat(import): move PDF parsing to backend; fix heartbeat/reap for import_tasks

- parsePDF function restored in import.go (body was orphaned outside function)
- ParseImportFile() called at upload time with 3-min timeout; chapters stored as JSON in MinIO
- runner.go: prefer ChaptersKey path (read pre-parsed JSON) over BookImport.Import()
- ImportChapterStore interface added; store wired in runner/main.go
- HeartbeatTask and ReapStaleTasks now include import_tasks collection
- parseImportTask now returns ChaptersKey in domain.ImportTask
- asynq_runner.go handleImportTask passes ChaptersKey
- pb-init-v3.sh: chapters_key field added to import_tasks schema

scripts/pb-init-v3.sh

@@ -306,6 +306,7 @@ create "import_tasks" '{
     {"name":"file_name",         "type":"text"},
     {"name":"file_type",         "type":"text"},
     {"name":"object_key",        "type":"text"},
+    {"name":"chapters_key",      "type":"text"},
     {"name":"author",            "type":"text"},
     {"name":"cover_url",         "type":"text"},
     {"name":"genres",            "type":"text"},