docs: add TODO.md

TODO.md

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+# TODO
+
+Known issues and deferred work. Agent-facing notes belong in the `AGENTS.md` files;
+this is for things that are broken or missing and not yet scheduled.
+
+## Backend
+
+### Identifier extraction drops most ISBNs
+
+`backend/src/chitai/services/metadata_extractor.py` — `EpubExtractor._extract_identifiers`
+
+The EPUB path validates `DC:identifier` values verbatim:
+
+```python
+for id in epub.get_metadata("DC", "identifier"):
+    if is_valid_isbn(id[0]):
+        ...
+```
+
+`is_valid_isbn` branches on `len(isbn)` being exactly 10 or 13, so anything carrying
+formatting fails. Two consequences:
+
+- **Hyphenated ISBNs are silently dropped.** `978-0-486-28211-4` is 17 characters, so it
+  never reaches the checksum. Most EPUBs write ISBNs hyphenated, so the majority are lost.
+  The PDF path already does this correctly — `_extract_isbns` calls
+  `match.replace("-", "")` before validating.
+- **`urn:isbn:` prefixes are dropped** for the same reason. This is a common EPUB form.
+
+Fix: normalise before validating — strip a leading `urn:isbn:`, then remove everything
+that isn't `0-9` or `X`. Reuse the PDF path's approach rather than duplicating it.
+
+Note this only runs at upload, so fixing it changes nothing for books already imported.
+A backfill would need to re-read the files on disk.
+
+### Non-ISBN identifiers are discarded
+
+Same function. Anything that isn't a valid ISBN is thrown away, including values EPUBs
+routinely carry: `urn:uuid:…`, `calibre:…`, Google Books volume IDs and ASINs.
+
+The `Identifier` model is already generic (`name` + `value`, unique per book), so storing
+them needs no schema change — only the extractor decides what survives. The frontend
+already renders ISBN, ASIN and DOI as links and shows unknown types as plain values, so
+anything stored will display sensibly.
+
+Worth adding at the same time:
+
+- A DOI regex (`10.\d{4,9}/\S+`) alongside the ISBN scan in `PdfExtractor._extract_isbns`
+  — academic PDFs carry one and it is the most useful identifier they have.
+- Deriving ISBN-10 from ISBN-13 when only the latter is present. It is a pure checksum
+  conversion and doubles the chance of an external lookup matching.
+
+## Frontend
+
+### Cover dimensions are unknown until load
+
+`book-cover.svelte` renders covers at a fixed height with natural width so nothing is
+cropped or distorted. Because the intrinsic size is not known ahead of time, the text
+beside the cover settles once when the image loads. `min-width` bounds the movement but
+does not remove it.
+
+Removing it properly means storing cover dimensions at ingest and emitting them as
+`width`/`height` attributes so the browser reserves exact space. That is a model change
+plus a migration.
+
+### No series navigation
+
+`Book` has `series` and `series_position`, and the detail page shows both, but there is no
+way to reach the other volumes. The books list endpoint filters by author, publisher, tag,
+shelf and progress — a `series` filter would need adding in
+`backend/src/chitai/services/filters/book.py` and wiring through
+`services/dependencies.py`, following the existing `AuthorFilter` pattern.
+
+### Book pages are sparse for most books
+
+EPUB metadata is thin: most imports arrive with a title, an author and nothing else. Two
+independent directions, neither started:
+
+- **Use what exists.** "More by this author" (the list endpoint already accepts
+  `authors=`), and exposing `Book.created_at` and `BookProgress.updated_at` — both are in
+  the database, neither is on `BookRead` / `BookProgressRead`.
+- **Fetch from outside.** Open Library or Google Books lookup by ISBN for descriptions and
+  covers. This is what actually fixes the sparseness, but it needs outbound requests, rate
+  limiting, a manual-vs-automatic decision, and a rule for not clobbering hand-edited
+  metadata.