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

src/server/models/config.py

@@ -2,16 +2,67 @@ from typing import Dict, List, Optional
 
 from pydantic import BaseModel, Field, ValidationError, field_validator
 
+_VALID_DAYS = frozenset(["mon", "tue", "wed", "thu", "fri", "sat", "sun"])
+_ALL_DAYS = ["mon", "tue", "wed", "thu", "fri", "sat", "sun"]
+
 
 class SchedulerConfig(BaseModel):
-    """Scheduler related configuration."""
+    """Scheduler related configuration.
+
+    Cron-based scheduling is configured via ``schedule_time`` and
+    ``schedule_days``.  The legacy ``interval_minutes`` field is kept for
+    backward compatibility but is **deprecated** and ignored when
+    ``schedule_time`` is set.
+    """
 
     enabled: bool = Field(
         default=True, description="Whether the scheduler is enabled"
     )
     interval_minutes: int = Field(
-        default=60, ge=1, description="Scheduler interval in minutes"
+        default=60,
+        ge=1,
+        description="[Deprecated] Scheduler interval in minutes. "
+                    "Use schedule_time + schedule_days instead.",
     )
+    schedule_time: str = Field(
+        default="03:00",
+        description="Daily run time in 24-hour HH:MM format (e.g. '03:00')",
+    )
+    schedule_days: List[str] = Field(
+        default_factory=lambda: list(_ALL_DAYS),
+        description="Days of week to run the scheduler (3-letter lowercase "
+                    "abbreviations: mon, tue, wed, thu, fri, sat, sun). "
+                    "Empty list means disabled.",
+    )
+    auto_download_after_rescan: bool = Field(
+        default=False,
+        description="Automatically queue and start downloads for all missing "
+                    "episodes after a scheduled rescan completes.",
+    )
+
+    @field_validator("schedule_time")
+    @classmethod
+    def validate_schedule_time(cls, v: str) -> str:
+        """Validate HH:MM format within 00:00–23:59."""
+        import re
+        if not re.fullmatch(r"([01]\d|2[0-3]):[0-5]\d", v or ""):
+            raise ValueError(
+                f"Invalid schedule_time '{v}'. "
+                "Expected HH:MM in 24-hour format (00:00–23:59)."
+            )
+        return v
+
+    @field_validator("schedule_days")
+    @classmethod
+    def validate_schedule_days(cls, v: List[str]) -> List[str]:
+        """Validate each entry is a valid 3-letter lowercase day abbreviation."""
+        invalid = [d for d in v if d not in _VALID_DAYS]
+        if invalid:
+            raise ValueError(
+                f"Invalid day(s) in schedule_days: {invalid}. "
+                f"Allowed values: {sorted(_VALID_DAYS)}"
+            )
+        return v
 
 
 class BackupConfig(BaseModel):