Add async video downloads with yt-dlp and Celery

- Added yt-dlp, celery, and redis dependencies to pyproject.toml
- Extended VideoEntry model with download tracking fields:
  - download_status (enum: pending, downloading, completed, failed)
  - download_path, download_started_at, download_completed_at
  - download_error, file_size
- Created celery_app.py with Redis broker configuration
- Created download_service.py with async download tasks:
  - download_video() task downloads as MP4 format
  - Configured yt-dlp for best MP4 quality with fallback
  - Automatic retries on failure (max 3 attempts)
  - Progress tracking and database updates
- Added Flask API endpoints in main.py:
  - POST /api/download/<video_id> to trigger download
  - GET /api/download/status/<video_id> to check status
  - POST /api/download/batch for bulk downloads
- Generated and applied Alembic migration for new fields
- Created downloads/ directory for video storage
- Updated .gitignore to exclude downloads/ directory
- Updated CLAUDE.md with comprehensive documentation:
  - Redis and Celery setup instructions
  - Download workflow and architecture
  - yt-dlp configuration details
  - New API endpoint examples

Videos are downloaded as MP4 files using Celery workers.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

models.py

@@ -1,12 +1,21 @@
 """Database models for YouTube feed storage."""
 
 from datetime import datetime
-from typing import List
+from typing import List, Optional
+from enum import Enum as PyEnum
 
-from sqlalchemy import String, DateTime, ForeignKey, Index
+from sqlalchemy import String, DateTime, ForeignKey, Index, Enum, BigInteger
 from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship
 
 
+class DownloadStatus(PyEnum):
+    """Download status enumeration."""
+    PENDING = "pending"
+    DOWNLOADING = "downloading"
+    COMPLETED = "completed"
+    FAILED = "failed"
+
+
 class Base(DeclarativeBase):
     """Base class for all database models."""
     pass
@@ -41,16 +50,29 @@ class VideoEntry(Base):
     link: Mapped[str] = mapped_column(String(500), unique=True, nullable=False)
     created_at: Mapped[datetime] = mapped_column(DateTime, nullable=False, default=datetime.utcnow)
 
+    # Download tracking fields
+    download_status: Mapped[DownloadStatus] = mapped_column(
+        Enum(DownloadStatus),
+        nullable=False,
+        default=DownloadStatus.PENDING
+    )
+    download_path: Mapped[Optional[str]] = mapped_column(String(1000), nullable=True)
+    download_started_at: Mapped[Optional[datetime]] = mapped_column(DateTime, nullable=True)
+    download_completed_at: Mapped[Optional[datetime]] = mapped_column(DateTime, nullable=True)
+    download_error: Mapped[Optional[str]] = mapped_column(String(2000), nullable=True)
+    file_size: Mapped[Optional[int]] = mapped_column(BigInteger, nullable=True)
+
     # Relationship to channel
     channel: Mapped["Channel"] = relationship("Channel", back_populates="videos")
 
     # Index for faster queries
     __table_args__ = (
         Index('idx_channel_created', 'channel_id', 'created_at'),
+        Index('idx_download_status', 'download_status'),
     )
 
     def __repr__(self) -> str:
-        return f"<VideoEntry(id={self.id}, title='{self.title}', link='{self.link}')>"
+        return f"<VideoEntry(id={self.id}, title='{self.title}', link='{self.link}', status='{self.download_status.value}')>"
 
     def to_dict(self) -> dict:
         """Convert to dictionary for API responses."""
@@ -58,5 +80,11 @@ class VideoEntry(Base):
             "id": self.id,
             "title": self.title,
             "link": self.link,
-            "created_at": self.created_at.isoformat()
+            "created_at": self.created_at.isoformat(),
+            "download_status": self.download_status.value,
+            "download_path": self.download_path,
+            "download_started_at": self.download_started_at.isoformat() if self.download_started_at else None,
+            "download_completed_at": self.download_completed_at.isoformat() if self.download_completed_at else None,
+            "download_error": self.download_error,
+            "file_size": self.file_size
         }