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/unit/test_filesystem_library.py

@@ -0,0 +1,69 @@
+"""Tests for BookPathGenerator."""
+
+from pathlib import Path
+
+from chitai.services.filesystem_library import BookPathGenerator, sanitize_path_component
+
+
+ROOT = Path("/library")
+
+
+def path_for(**book) -> Path:
+    return BookPathGenerator(ROOT).generate_path(book)
+
+
+def test_author_and_title() -> None:
+    assert path_for(title="Dune", authors=["Frank Herbert"]) == (
+        ROOT / "Frank Herbert" / "Dune"
+    )
+
+
+def test_a_book_with_no_authors() -> None:
+    assert path_for(title="Beowulf", authors=[]) == ROOT / "Unknown" / "Beowulf"
+
+
+def test_a_series_adds_a_level_and_pads_the_position() -> None:
+    assert path_for(
+        title="Persepolis Rising",
+        authors=["James S. A. Corey"],
+        series="The Expanse",
+        series_position="7",
+    ) == ROOT / "James S. A. Corey" / "The Expanse" / "07 - Persepolis Rising"
+
+
+def test_a_slash_in_a_title_does_not_add_a_directory() -> None:
+    """
+    The separators in the path come from the template, never from the metadata.
+
+    A title with a slash in it — "AC/DC", "Him/Her" — would otherwise put the book one
+    level below where `book.path` says it is, which is what deletes, moves and file
+    lookups all act on. Calibre keeps the real title in its database and strips this
+    from its own directory names, so an import is where they surface.
+    """
+    generated = path_for(title="Back in Black: AC/DC", authors=["Murray Engleheart"])
+
+    assert generated == ROOT / "Murray Engleheart" / "Back in Black: AC_DC"
+    assert generated.relative_to(ROOT).parts == ("Murray Engleheart", "Back in Black: AC_DC")
+
+
+def test_a_slash_in_an_author_or_series_is_handled_too() -> None:
+    assert path_for(title="Split", authors=["A/B Collective"]) == (
+        ROOT / "A_B Collective" / "Split"
+    )
+    assert path_for(
+        title="Volume One", authors=["Someone"], series="Either/Or", series_position="1"
+    ) == ROOT / "Someone" / "Either_Or" / "01 - Volume One"
+
+
+def test_control_characters_are_removed() -> None:
+    assert path_for(title="Line\nBreak", authors=["Someone"]) == (
+        ROOT / "Someone" / "Line_Break"
+    )
+
+
+def test_sanitize_path_component() -> None:
+    assert sanitize_path_component("AC/DC") == "AC_DC"
+    assert sanitize_path_component("back\\slash") == "back_slash"
+    assert sanitize_path_component("  padded  ") == "padded"
+    # Colons and other punctuation are legal in a path and are left alone.
+    assert sanitize_path_component("Title: Subtitle") == "Title: Subtitle"