Add queue deduplication to prevent duplicate entries

- In-memory dedup in add_to_queue() using _pending_by_episode dict
- Batch-local dedup via seen_in_batch set (handles duplicates within single call)
- Database unique index on episode_id via __table_args__
- 5-minute cooldown in _auto_download_missing() to prevent rapid re-triggers
- Updated _add_to_pending_queue() and _remove_from_pending_queue() to track episode keys
- Added TestQueueDeduplication with 4 test cases
- Updated DEVELOPMENT.md and TESTING.md with queue dedup docs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/server/services/scheduler_service.py

@@ -44,6 +44,9 @@ class SchedulerService:
         self._config: Optional[SchedulerConfig] = None
         self._last_scan_time: Optional[datetime] = None
         self._scan_in_progress: bool = False
+        # Cooldown tracking for auto-download to prevent rapid re-triggers
+        self._last_auto_download_time: Optional[datetime] = None
+        self._auto_download_cooldown_seconds: int = 300  # 5 minutes default
         logger.info("SchedulerService initialised")
 
     # ------------------------------------------------------------------
@@ -256,12 +259,26 @@ class SchedulerService:
 
     async def _auto_download_missing(self) -> None:
         """Queue and start downloads for all series with missing episodes."""
+        from datetime import timedelta  # noqa: PLC0415
+
         from src.server.models.download import EpisodeIdentifier  # noqa: PLC0415
         from src.server.utils.dependencies import (  # noqa: PLC0415
             get_anime_service,
             get_download_service,
         )
 
+        # Check cooldown to prevent rapid re-triggers
+        now = datetime.now(timezone.utc)
+        if self._last_auto_download_time is not None:
+            elapsed = now - self._last_auto_download_time
+            if elapsed < timedelta(seconds=self._auto_download_cooldown_seconds):
+                logger.debug(
+                    "Auto-download skipped: cooldown active",
+                    elapsed_seconds=elapsed.total_seconds(),
+                    cooldown_seconds=self._auto_download_cooldown_seconds,
+                )
+                return
+
         anime_service = get_anime_service()
         download_service = get_download_service()
 
@@ -303,6 +320,9 @@ class SchedulerService:
         await self._broadcast("auto_download_started", {"queued_count": queued_count})
         logger.info("Auto-download completed", queued_count=queued_count)
 
+        # Update cooldown timestamp after successful auto-download
+        self._last_auto_download_time = datetime.now(timezone.utc)
+
     async def _perform_rescan(self) -> None:
         """Execute a library rescan and optionally trigger auto-download."""
         if self._scan_in_progress: