Case-insensitive login, auto-matching after scan, per-library sources

- auth: username lookup is now case-insensitive via func.lower()
- scanner: trigger match_audiobook for each newly found item after scan
- matcher: read match_sources from library settings; refactored to loop
  over configured sources in priority order instead of hardcoded sequence
- schemas/routers: expose matchSources in LibraryOut API response
- Admin UI: pill-toggle for MusicBrainz/Open Library/Google Books per library

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/auth.py

@@ -1,7 +1,7 @@
 from datetime import datetime
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select
+from sqlalchemy import select, func
 from ..database import AsyncSessionLocal
 from ..dependencies import get_db, get_current_user
 from ..models.user import User
@@ -42,7 +42,7 @@ def _build_user_out(user: User) -> UserOut:
 @router.post("/login", response_model=LoginResponse)
 async def login(body: LoginRequest, db: AsyncSession = Depends(get_db)):
     result = await db.execute(
-        select(User).where(User.username == body.username, User.is_active == True)
+        select(User).where(func.lower(User.username) == body.username.lower(), User.is_active == True)
     )
     user = result.scalar_one_or_none()
     if not user or not verify_password(body.password, user.password_hash):