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/migrations/versions/2026-08-17_backfill_file_content_types_d2d69065ede3.py

@@ -0,0 +1,114 @@
+"""backfill file content types
+
+Revision ID: d2d69065ede3
+Revises: 49a9e85a0ffc
+Create Date: 2026-08-17 11:37:58.959135
+
+"""
+
+import warnings
+from typing import TYPE_CHECKING
+
+import sqlalchemy as sa
+from alembic import op
+from advanced_alchemy.types import EncryptedString, EncryptedText, GUID, ORA_JSONB, DateTimeUTC, StoredObject, PasswordHash, FernetBackend
+from advanced_alchemy.types.encrypted_string import PGCryptoBackend
+from advanced_alchemy.types.password_hash.argon2 import Argon2Hasher
+from advanced_alchemy.types.password_hash.passlib import PasslibHasher
+from advanced_alchemy.types.password_hash.pwdlib import PwdlibHasher
+from sqlalchemy import Text  # noqa: F401
+
+if TYPE_CHECKING:
+    from collections.abc import Sequence
+
+__all__ = ["downgrade", "upgrade", "schema_upgrades", "schema_downgrades", "data_upgrades", "data_downgrades"]
+
+sa.GUID = GUID
+sa.DateTimeUTC = DateTimeUTC
+sa.ORA_JSONB = ORA_JSONB
+sa.EncryptedString = EncryptedString
+sa.EncryptedText = EncryptedText
+sa.StoredObject = StoredObject
+sa.PasswordHash = PasswordHash
+sa.Argon2Hasher = Argon2Hasher
+sa.PasslibHasher = PasslibHasher
+sa.PwdlibHasher = PwdlibHasher
+sa.FernetBackend = FernetBackend
+sa.PGCryptoBackend = PGCryptoBackend
+
+# revision identifiers, used by Alembic.
+revision = 'd2d69065ede3'
+down_revision = '49a9e85a0ffc'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    with warnings.catch_warnings():
+        warnings.filterwarnings("ignore", category=UserWarning)
+        with op.get_context().autocommit_block():
+            schema_upgrades()
+            data_upgrades()
+
+def downgrade() -> None:
+    with warnings.catch_warnings():
+        warnings.filterwarnings("ignore", category=UserWarning)
+        with op.get_context().autocommit_block():
+            data_downgrades()
+            schema_downgrades()
+
+def schema_upgrades() -> None:
+    """schema upgrade migrations go here."""
+    pass
+
+def schema_downgrades() -> None:
+    """schema downgrade migrations go here."""
+    pass
+
+def data_upgrades() -> None:
+    """
+    Name the format of every file whose content type was never worked out.
+
+    `create_many_from_existing_files` filled the column from `mimetypes.guess_type`,
+    which answers None for `.mobi`, `.azw`, `.fb2` and `.lit` — so a consume-directory
+    import of any of those stored a null, and the OPDS acquisition link a reader app
+    uses to decide what it can open carried nothing.
+
+    Both write paths now go through `guess_content_type`, which this uses too, so the
+    formats in its table get named retroactively. A row it still cannot name is **left
+    null** rather than filled with a placeholder: null is the truth, the column is
+    nullable, and the one consumer that needs a string substitutes one itself.
+    Idempotent: it only looks at rows that carry nothing.
+    """
+    from chitai.services.utils import guess_content_type
+
+    connection = op.get_bind()
+
+    files = connection.execute(
+        sa.text(
+            "SELECT id, path FROM file_metadata "
+            "WHERE content_type IS NULL OR content_type = ''"
+        )
+    ).fetchall()
+
+    parameters = [
+        {"id": id, "content_type": content_type}
+        for id, path in files
+        if (content_type := guess_content_type(path)) is not None
+    ]
+
+    if not parameters:
+        return
+
+    batch_size = 1000
+    for start in range(0, len(parameters), batch_size):
+        connection.execute(
+            sa.text(
+                "UPDATE file_metadata SET content_type = :content_type WHERE id = :id"
+            ),
+            parameters[start : start + batch_size],
+        )
+
+
+def data_downgrades() -> None:
+    """Add any optional data downgrade migrations here!"""