Per discussion #1372, where you said you were fine with this specific
implementation: check whether metadata.db exists, read it if so, and
show a check mark saying the book is already there.
Searching for a book you already own gives no hint that you own it, so
the easiest way to end up with a second copy is to not remember you have
the first. This reads a Calibre `metadata.db`, read only, and marks
matching results with an **In library** badge in the card, list and
compact views and in the details dialog.
Off by default. It sits in Settings, General beside the existing Library
URL, with a test button that reports how many books it indexed. No HTTP
call, no token, nothing written back.
Matching runs most to least confident: a shared external id, then an
ISBN compared in both ISBN-10 and ISBN-13 form, then fuzzy title tokens
plus the author surname. The check fails open, so an unreadable database
degrades the badge and never blocks a search, and entries are cached for
ten minutes with an early refresh when the file changes, so a large
library costs one read rather than one per search.
`text_match.py` is new and shared by the index and the provider, so
title, author and ISBN matching stays consistent in one place.
## On the provider interface
`library_index` talks only to a `LibraryProvider` protocol and knows
nothing about Calibre. That is deliberate but it is not speculative
generality, it is what let me send you the Calibre half on its own: I
run an Audiobookshelf provider on the same interface in my fork, which
is where the audiobook side of the badge comes from. I have left that
out because it is a new service integration rather than something
already in the codebase, which is the line your non-goals draw. Happy to
send it separately if you ever want it, and equally happy for the answer
to be no.
Adding a library is a module with the `LibraryProvider` shape plus one
line in `all_providers()`.
## Verification
- `tests/core/test_library_index.py`: id, ISBN and fuzzy matching,
per-content-type provider selection, fail-open on provider errors,
stale-cache reuse, TTL and fingerprint refresh, per-provider cache
isolation, and the test-connection path including unsaved form values.
- `tests/core/test_text_match.py`: ISBN variants and token matching.
- `src/frontend/src/tests/libraryBadge.test.ts` and the added cases in
`bookTransformers.test.ts`.
- Python suite (3269) and frontend suite (206) green, plus ruff, ruff
format, basedpyright, vulture, tsc, oxlint, oxfmt and the production
build.
