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/storage.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+import hashlib
+from pathlib import Path
+
+from app.config import settings
+
+
+class NasContentStorage:
+    def __init__(self, root_dir: str):
+        self.root = Path(root_dir).resolve()
+        self.root.mkdir(parents=True, exist_ok=True)
+
+    def _resolve(self, href: str) -> Path:
+        rel = href.strip().lstrip("/")
+        target = (self.root / rel).resolve()
+        if self.root not in target.parents and target != self.root:
+            raise ValueError("Invalid storage href")
+        return target
+
+    def read_text(self, href: str) -> str:
+        path = self._resolve(href)
+        return path.read_text(encoding="utf-8")
+
+    def write_text(self, href: str, content: str) -> dict[str, str | int]:
+        path = self._resolve(href)
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(content, encoding="utf-8")
+        digest = hashlib.sha256(content.encode("utf-8")).hexdigest()
+        return {"href": href, "sha256": digest, "size": len(content.encode("utf-8"))}
+
+
+storage = NasContentStorage(settings.nas_content_root)