feat(storage): implement NAS content storage with read/write capabilities

feat(docker): configure NAS content and EPUB source directories in docker-compose

feat(migrations): add tables for SourceAsset, ImportJob, ChapterContentRef, and AssetNovelMapping

feat(scripts): create backfill script for populating ChapterContentRef from MongoDB chapters

app/config.py

@@ -20,6 +20,9 @@ class Settings(BaseSettings):
     r2_secret_access_key: str = ""
     r2_bucket_name: str = ""
     r2_public_base_url: str = ""
+    nas_content_root: str = "./data/content"
+    epub_source_root: str = "./data/epub-source"
+    chapter_content_mode: str = "nas_first"  # nas_first | mongo_first
 
     deepseek_key: str = ""
     deepseek_model: str = "deepseek-chat"