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/database/models.py

@@ -15,7 +15,7 @@ from datetime import datetime, timezone
 from enum import Enum
 from typing import List, Optional
 
-from sqlalchemy import Boolean, DateTime, ForeignKey, Integer, String, Text, func
+from sqlalchemy import Boolean, DateTime, ForeignKey, Index, Integer, String, Text, func
 from sqlalchemy.orm import Mapped, mapped_column, relationship, validates
 
 from src.server.database.base import Base, TimestampMixin
@@ -347,6 +347,16 @@ class DownloadQueueItem(Base, TimestampMixin):
         index=True
     )
     
+    # Unique constraint to prevent duplicate pending queue items
+    # An episode can only have one queue entry at a time
+    __table_args__ = (
+        Index(
+            "ix_download_queue_episode_pending",
+            "episode_id",
+            unique=True,
+        ),
+    )
+    
     # Error handling
     error_message: Mapped[Optional[str]] = mapped_column(
         Text, nullable=True,