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

ROLLOUT_CHECKLIST.md

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+# Rollout Checklist - NAS Chapter Storage
+
+## Pre-Deploy
+- [ ] Backup PostgreSQL schema + critical tables
+- [ ] Verify NAS mount/access permissions in API runtime
+- [ ] Enable feature flags (default: Mongo fallback on)
+
+## Deploy Order
+1. Deploy DB migrations
+2. Deploy API with dual-read disabled by default
+3. Enable discover/approve/convert job APIs
+4. Run pilot import set (small curated EPUB batch)
+5. Enable NAS-first for pilot users/env
+6. Gradually ramp NAS-first traffic
+
+## Runtime Verification
+- [ ] `/api/health` stable
+- [ ] Chapter read success rate >= target
+- [ ] NAS read timeout/error rate below threshold
+- [ ] Mongo fallback rate trending down
+
+## Rollback
+- [ ] Switch feature flag to Mongo-first immediately
+- [ ] Stop import jobs
+- [ ] Keep imported refs for investigation (no destructive cleanup)
+
+## Post-Deploy
+- [ ] Compare chapter counts and random content samples
+- [ ] Review failed/review_required import queue
+- [ ] Publish release notes for web/mobile teams