Add comprehensive test suite for backend (133 tests)

Test coverage:
- test_utils.py: formatting helpers (human_size, ticks_to_minutes,
  format_duration, format_bitrate, ffprobe summaries, stream parsing)
- test_path_utils.py: path resolution (prefix, media root mapping,
  edge cases with spaces/special chars)
- test_domain_media.py: Jellyfin item normalization (HDR detection,
  media sources, stream extraction, display formatting)
- test_jobs.py: job template rendering and shell quoting safety
- test_media_index.py: SQLite index CRUD, querying, filtering,
  sorting, pagination
- test_config.py: pydantic-settings env loading
- test_api.py: full FastAPI integration tests with mocked SSH/Jellyfin
  (all endpoints: dashboard, monitoring, media, files, jobs)

All tests run without network/SSH dependencies using mocks.

backend/src/media_library_viewer_api/dependencies.py

@@ -17,7 +17,7 @@ from media_library_viewer_api.config import get_settings
 def get_jellyfin_client() -> JellyfinClient:
     """Return a cached Jellyfin client."""
     settings = get_settings()
-    return JellyfinClient(settings.jellyfin.url, settings.jellyfin.api_key)
+    return JellyfinClient(settings.jellyfin_url, settings.jellyfin_api_key)
 
 
 @lru_cache
@@ -25,11 +25,11 @@ def get_ssh_client() -> RemoteSSHClient:
     """Return a cached SSH client (connects on first use)."""
     settings = get_settings()
     client = RemoteSSHClient(
-        host=settings.ssh.host,
-        username=settings.ssh.username,
-        port=settings.ssh.port,
-        key_filename=settings.ssh.key_filename or None,
-        password=settings.ssh.password or None,
+        host=settings.ssh_host,
+        username=settings.ssh_username,
+        port=settings.ssh_port,
+        key_filename=settings.ssh_key_filename or None,
+        password=settings.ssh_password or None,
     )
     client.connect()
     return client
@@ -38,8 +38,8 @@ def get_ssh_client() -> RemoteSSHClient:
 def get_user_id() -> str:
     """Return the configured Jellyfin user ID, or discover the first available user."""
     settings = get_settings()
-    if settings.jellyfin.user_id:
-        return settings.jellyfin.user_id
+    if settings.jellyfin_user_id:
+        return settings.jellyfin_user_id
     client = get_jellyfin_client()
     users = client.users()
     if not users: