feat: cron-based scheduler with auto-download after rescan

- Replace asyncio sleep loop with APScheduler AsyncIOScheduler + CronTrigger
- Add schedule_time (HH:MM), schedule_days (days of week), auto_download_after_rescan fields to SchedulerConfig
- Add _auto_download_missing() to queue missing episodes after rescan
- Reload config live via reload_config(SchedulerConfig) without restart
- Update GET/POST /api/scheduler/config to return {success, config, status} envelope
- Add day-of-week pill toggles to Settings -> Scheduler section in UI
- Update JS loadSchedulerConfig / saveSchedulerConfig for new API shape
- Add 29 unit tests for SchedulerConfig model, 18 unit tests for SchedulerService
- Rewrite 23 endpoint tests and 36 integration tests for APScheduler behaviour
- Coverage: 96% api/scheduler, 95% scheduler_service, 90% total (>= 80% threshold)
- Update docs: API.md, CONFIGURATION.md, features.md, CHANGELOG.md

tests/unit/test_scheduler_config_model.py

@@ -0,0 +1,129 @@
+"""Unit tests for SchedulerConfig model fields and validators (Task 3)."""
+import pytest
+from pydantic import ValidationError
+
+from src.server.models.config import SchedulerConfig
+
+ALL_DAYS = ["mon", "tue", "wed", "thu", "fri", "sat", "sun"]
+
+
+class TestSchedulerConfigDefaults:
+    """3.1 – Default values."""
+
+    def test_default_schedule_time(self) -> None:
+        config = SchedulerConfig()
+        assert config.schedule_time == "03:00"
+
+    def test_default_schedule_days(self) -> None:
+        config = SchedulerConfig()
+        assert config.schedule_days == ALL_DAYS
+
+    def test_default_auto_download(self) -> None:
+        config = SchedulerConfig()
+        assert config.auto_download_after_rescan is False
+
+    def test_default_enabled(self) -> None:
+        config = SchedulerConfig()
+        assert config.enabled is True
+
+    def test_default_interval_minutes(self) -> None:
+        config = SchedulerConfig()
+        assert config.interval_minutes == 60
+
+
+class TestSchedulerConfigValidScheduleTime:
+    """3.2 – Valid schedule_time values."""
+
+    @pytest.mark.parametrize("time_val", ["00:00", "03:00", "12:30", "23:59"])
+    def test_valid_times(self, time_val: str) -> None:
+        config = SchedulerConfig(schedule_time=time_val)
+        assert config.schedule_time == time_val
+
+
+class TestSchedulerConfigInvalidScheduleTime:
+    """3.3 – Invalid schedule_time values must raise ValidationError."""
+
+    @pytest.mark.parametrize(
+        "time_val",
+        ["25:00", "3pm", "", "3:00pm", "24:00", "-1:00", "9:00", "1:60"],
+    )
+    def test_invalid_times(self, time_val: str) -> None:
+        with pytest.raises(ValidationError):
+            SchedulerConfig(schedule_time=time_val)
+
+
+class TestSchedulerConfigValidScheduleDays:
+    """3.4 – Valid schedule_days values."""
+
+    def test_single_day(self) -> None:
+        config = SchedulerConfig(schedule_days=["mon"])
+        assert config.schedule_days == ["mon"]
+
+    def test_multiple_days(self) -> None:
+        config = SchedulerConfig(schedule_days=["mon", "fri"])
+        assert config.schedule_days == ["mon", "fri"]
+
+    def test_all_days(self) -> None:
+        config = SchedulerConfig(schedule_days=ALL_DAYS)
+        assert config.schedule_days == ALL_DAYS
+
+    def test_empty_list(self) -> None:
+        config = SchedulerConfig(schedule_days=[])
+        assert config.schedule_days == []
+
+
+class TestSchedulerConfigInvalidScheduleDays:
+    """3.5 – Invalid schedule_days values must raise ValidationError."""
+
+    @pytest.mark.parametrize(
+        "days",
+        [
+            ["monday"],
+            ["xyz"],
+            ["Mon"],      # Case-sensitive — must be lowercase
+            [""],
+        ],
+    )
+    def test_invalid_days(self, days: list) -> None:
+        with pytest.raises(ValidationError):
+            SchedulerConfig(schedule_days=days)
+
+
+class TestSchedulerConfigAutoDownload:
+    """3.6 – auto_download_after_rescan field."""
+
+    def test_set_true(self) -> None:
+        config = SchedulerConfig(auto_download_after_rescan=True)
+        assert config.auto_download_after_rescan is True
+
+    def test_set_false(self) -> None:
+        config = SchedulerConfig(auto_download_after_rescan=False)
+        assert config.auto_download_after_rescan is False
+
+
+class TestSchedulerConfigBackwardCompat:
+    """3.7 – Backward compatibility: old fields still work."""
+
+    def test_legacy_fields_use_defaults(self) -> None:
+        config = SchedulerConfig(enabled=True, interval_minutes=30)
+        assert config.schedule_time == "03:00"
+        assert config.schedule_days == ALL_DAYS
+        assert config.auto_download_after_rescan is False
+        assert config.enabled is True
+        assert config.interval_minutes == 30
+
+
+class TestSchedulerConfigSerialisation:
+    """3.8 – Serialisation roundtrip."""
+
+    def test_roundtrip(self) -> None:
+        original = SchedulerConfig(
+            enabled=True,
+            interval_minutes=120,
+            schedule_time="04:30",
+            schedule_days=["mon", "wed", "fri"],
+            auto_download_after_rescan=True,
+        )
+        dumped = original.model_dump()
+        restored = SchedulerConfig(**dumped)
+        assert restored == original