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/tests/test_config.py

@@ -0,0 +1,45 @@
+"""Unit tests for config.py settings loading."""
+
+import os
+from unittest.mock import patch
+from media_library_viewer_api.config import Settings
+
+
+class TestSettings:
+    def test_defaults(self):
+        with patch.dict(os.environ, {}, clear=True):
+            settings = Settings(_env_file=None)
+            assert settings.ssh_host == ""
+            assert settings.ssh_port == 22
+            assert settings.jellyfin_url == ""
+            assert settings.remote_media_root == ""
+
+    def test_from_env(self):
+        env = {
+            "JELLYFIN_URL": "https://test.example.com",
+            "JELLYFIN_API_KEY": "key123",
+            "SSH_HOST": "192.168.1.1",
+            "SSH_USERNAME": "testuser",
+            "SSH_PORT": "2222",
+            "REMOTE_MEDIA_ROOT": "/srv/media",
+        }
+        with patch.dict(os.environ, env, clear=True):
+            settings = Settings(_env_file=None)
+            assert settings.jellyfin_url == "https://test.example.com"
+            assert settings.jellyfin_api_key == "key123"
+            assert settings.ssh_host == "192.168.1.1"
+            assert settings.ssh_username == "testuser"
+            assert settings.ssh_port == 2222
+            assert settings.remote_media_root == "/srv/media"
+
+    def test_media_root_property(self):
+        env = {"REMOTE_MEDIA_ROOT": "/mnt/data"}
+        with patch.dict(os.environ, env, clear=True):
+            settings = Settings(_env_file=None)
+            assert settings.media_root == "/mnt/data"
+
+    def test_path_prefix_property(self):
+        env = {"REMOTE_PATH_PREFIX": "/srv"}
+        with patch.dict(os.environ, env, clear=True):
+            settings = Settings(_env_file=None)
+            assert settings.path_prefix == "/srv"