Cache library index and refresh changed manga

docs/import.md

@@ -4,11 +4,12 @@ MangaCircus does not copy or convert anything. "Import" means **indexing the lib
 
 ```
                  ┌───────────── triggers ─────────────┐
-                 │ start-up · every SCAN_INTERVAL min │
+                 │ start-up · shallow interval · daily full │
                  │ admin "Rescan" · admin "Full rescan"│
                  └────────────────┬───────────────────┘
                                   ▼
- library/ ──read──▶ scanLibrary(root, oldCache)            library.js
+ data/library-index.json ──▶ immediate in-memory index at start (if valid)
+ library/ ──read──▶ shallow changed-manga check or full scan   library.js
                       ├─ scanManga      (per manga folder)
                       │    ├─ book folders  → parseKey, book.json, cover
                       │    └─ chapter folders → scanVersion
@@ -20,19 +21,23 @@ MangaCircus does not copy or convert anything. "Import" means **indexing the lib
                                   ▼
                       lib = index       (atomic swap, requests see old or new, never half)
                                   ▼
-                      data/scan-cache.json   (written atomically)
+                      data/scan-cache.json + data/library-index.json (atomic)
 ```
 
 ## 1. Triggers
 
 | Trigger | When | Cache |
 |---------|------|-------|
-| Start-up | Right after the HTTP server starts listening. Requests are served meanwhile; the library page shows "Scanning…" until the first scan is done | used |
-| Interval | Every `SCAN_INTERVAL` minutes (default 10, `0` = off) | used |
+| Start-up | Restore the saved index immediately if its library mount is present, then check changed manga. Without a valid snapshot, scan the whole library; the page shows "Scanning…" until done | used |
+| Interval | Shallow change check every `SCAN_INTERVAL` minutes (default 10, `0` = off); full reconciliation every `FULL_SCAN_INTERVAL` minutes (default 1440, `0` = off) | used |
 | **Rescan** (Settings → Admin, or `POST /api/admin/rescan {}`) | On demand; the request returns when the scan is done | used |
 | **Full rescan** (`POST /api/admin/rescan {"full":true}`) | On demand | ignored: every image header is read again |
 
-Scans are **single-flight**. A trigger that fires while a scan is running joins that scan instead of starting a second one.
+Scans are **single-flight**. A trigger during a scan queues one coalesced follow-up,
+with a full reconciliation taking precedence over a shallow check. Publication
+revisions trigger a shallow check; if no changed directory is visible, the reader
+falls back to a full scan. In-place edits to `chapter.json` are found by the full
+reconciliation or an admin Rescan, not by shallow parent-directory stamps.
 
 ## 2. Walking the tree
 
@@ -155,6 +160,11 @@ The reader lays pages out before they load, so it needs every image's size. Inst
 ## 4. The scan cache
 
 - Stored in `DATA/scan-cache.json`. It is rewritten atomically after every successful scan and holds only the folders seen in that scan, so deleted chapters drop out.
+- `DATA/library-index.json` also stores the complete rebuildable index and shallow
+  manga/book directory stamps. It lets a restarted reader serve the dashboard and
+  manga pages before the background filesystem check completes. Both files can be
+  deleted safely; without the index the next startup walks the library, and without
+  the size cache it also rereads image headers.
 - Invalidation is **per chapter folder, by its mtime**. Adding, removing or renaming a file changes the folder mtime, so that folder is re-read on the next scan.
 - **Blind spot:** overwriting an image *in place* (same name) usually doesn't change the folder's mtime, so the old size stays cached. Run a **Full rescan** after such edits, or write new files and rename them (§7).
 - Deleting the cache file is harmless. The next scan is simply slower.