feat: import a Calibre library

Reads metadata.db and copies the books into a library — from a zip uploaded on
the library settings page, or from a path with `litestar calibre-import`. The
source is never touched, and re-running only picks up what is new.

Also names the formats mimetypes does not know: a Calibre library is full of
MOBI and AZW3, and a null content type used to fail the book endpoint.

backend/tests/integration/test_book.py

@@ -1272,3 +1272,94 @@ class TestFileManagement:
         )
         # Should succeed (idempotent)
         assert response2.status_code == 204
+
+
+@pytest.mark.asyncio
+class TestUnnameableFormats:
+    """
+    A file whose format nothing can name must still round-trip.
+
+    `mimetypes.guess_type` answers None for `.mobi`, `.azw`, `.fb2` and `.lit`, which
+    is most of what a library imported from elsewhere carries alongside its EPUBs.
+    `FileMetadataRead.content_type` used to be a required string, so such a book was
+    created and then failed serialisation on its way back out — a 500 on a book the
+    reader can otherwise download.
+    """
+
+    def upload(self, name: str) -> list[tuple[str, tuple]]:
+        # `application/octet-stream` is what a browser posts for these, and it is not
+        # an answer — the extension is what names the format.
+        return [("files", (name, b"BOOKMOBI\x00 payload", "application/octet-stream"))]
+
+    async def test_a_mobi_is_named_from_its_extension(
+        self, authenticated_client: AsyncClient
+    ) -> None:
+        response = await authenticated_client.post(
+            "/books?library_id=1", files=self.upload("Dune.mobi"), data={"library_id": 1}
+        )
+
+        assert response.status_code == 201
+        book = response.json()
+        assert book["files"][0]["content_type"] == "application/x-mobipocket-ebook"
+
+        detail = await authenticated_client.get(f"/books/{book['id']}")
+        assert detail.status_code == 200
+
+    async def test_an_unknown_extension_stores_no_content_type(
+        self, authenticated_client: AsyncClient
+    ) -> None:
+        """Null, not a placeholder — and the book still serialises either way."""
+        response = await authenticated_client.post(
+            "/books?library_id=1",
+            files=self.upload("Notes.xyzzy"),
+            data={"library_id": 1},
+        )
+
+        assert response.status_code == 201
+        book = response.json()
+        assert book["files"][0]["content_type"] is None
+
+        detail = await authenticated_client.get(f"/books/{book['id']}")
+        assert detail.status_code == 200
+        assert detail.json()["files"][0]["content_type"] is None
+
+    async def test_the_file_downloads(
+        self, authenticated_client: AsyncClient
+    ) -> None:
+        """Litestar supplies its own media type when the row carries none."""
+        created = await authenticated_client.post(
+            "/books?library_id=1",
+            files=self.upload("Notes.xyzzy"),
+            data={"library_id": 1},
+        )
+        book = created.json()
+
+        response = await authenticated_client.get(
+            f"/books/download/{book['id']}/{book['files'][0]['id']}"
+        )
+
+        assert response.status_code == 200
+        assert response.headers["content-type"] == "application/octet-stream"
+
+    async def test_the_opds_feed_survives_a_null_content_type(
+        self, authenticated_client: AsyncClient
+    ) -> None:
+        """
+        The one place the type has to be a string.
+
+        `Link.type` is required, so a null fails the whole feed rather than one entry.
+        OPDS clients speak Basic, not the JWT the rest of the API uses.
+        """
+        await authenticated_client.post(
+            "/books?library_id=1",
+            files=self.upload("Notes.xyzzy"),
+            data={"library_id": 1},
+        )
+
+        feed = await authenticated_client.get(
+            "/opds/acquisition?feed_id=all&feed_title=All+Books",
+            auth=("user1@example.com", "password123"),
+        )
+
+        assert feed.status_code == 200
+        assert 'type="application/octet-stream"' in feed.text