Fix multi-worker temp file issue with file-based storage

- New temp_storage.py module stores files on disk instead of in-memory
- Multiple Gunicorn workers can now share temp files
- Startup cleanup removes leftover files from previous runs
- Dockerfile creates temp_files directory
- Added temp_files/ to .gitignore

Previously encode preview worked but download failed with "File expired"
because each worker had its own in-memory TEMP_FILES dict.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/temp_storage.py

@@ -0,0 +1,214 @@
+"""
+File-based Temporary Storage
+
+Stores temp files on disk instead of in-memory dict.
+This allows multiple Gunicorn workers to share temp files
+and survives service restarts within the expiry window.
+
+Files are stored in a temp directory with:
+- {file_id}.data - The actual file data
+- {file_id}.json - Metadata (filename, timestamp, mime_type, etc.)
+
+IMPORTANT: This module ONLY manages files in the temp_files/ directory.
+It does NOT touch instance/ (auth database) or any other directories.
+"""
+
+import json
+import os
+import shutil
+import time
+from pathlib import Path
+from threading import Lock
+
+# Default temp directory (can be overridden)
+DEFAULT_TEMP_DIR = Path(__file__).parent / "temp_files"
+
+# Lock for thread-safe operations
+_lock = Lock()
+
+# Module-level temp directory (set on init)
+_temp_dir: Path = DEFAULT_TEMP_DIR
+
+
+def init(temp_dir: Path | str | None = None):
+    """Initialize temp storage with optional custom directory."""
+    global _temp_dir
+    _temp_dir = Path(temp_dir) if temp_dir else DEFAULT_TEMP_DIR
+    _temp_dir.mkdir(parents=True, exist_ok=True)
+
+
+def _data_path(file_id: str) -> Path:
+    """Get path for file data."""
+    return _temp_dir / f"{file_id}.data"
+
+
+def _meta_path(file_id: str) -> Path:
+    """Get path for file metadata."""
+    return _temp_dir / f"{file_id}.json"
+
+
+def _thumb_path(thumb_id: str) -> Path:
+    """Get path for thumbnail data."""
+    return _temp_dir / f"{thumb_id}.thumb"
+
+
+def save_temp_file(file_id: str, data: bytes, metadata: dict) -> None:
+    """
+    Save a temp file with its metadata.
+
+    Args:
+        file_id: Unique identifier for the file
+        data: File contents as bytes
+        metadata: Dict with filename, mime_type, timestamp, etc.
+    """
+    init()  # Ensure directory exists
+
+    with _lock:
+        # Add timestamp if not present
+        if "timestamp" not in metadata:
+            metadata["timestamp"] = time.time()
+
+        # Write data file
+        _data_path(file_id).write_bytes(data)
+
+        # Write metadata
+        _meta_path(file_id).write_text(json.dumps(metadata))
+
+
+def get_temp_file(file_id: str) -> dict | None:
+    """
+    Get a temp file and its metadata.
+
+    Returns:
+        Dict with 'data' (bytes) and all metadata fields, or None if not found.
+    """
+    init()
+
+    data_file = _data_path(file_id)
+    meta_file = _meta_path(file_id)
+
+    if not data_file.exists() or not meta_file.exists():
+        return None
+
+    try:
+        data = data_file.read_bytes()
+        metadata = json.loads(meta_file.read_text())
+        return {"data": data, **metadata}
+    except (OSError, json.JSONDecodeError):
+        return None
+
+
+def has_temp_file(file_id: str) -> bool:
+    """Check if a temp file exists."""
+    init()
+    return _data_path(file_id).exists() and _meta_path(file_id).exists()
+
+
+def delete_temp_file(file_id: str) -> None:
+    """Delete a temp file and its metadata."""
+    init()
+
+    with _lock:
+        _data_path(file_id).unlink(missing_ok=True)
+        _meta_path(file_id).unlink(missing_ok=True)
+
+
+def save_thumbnail(thumb_id: str, data: bytes) -> None:
+    """Save a thumbnail."""
+    init()
+
+    with _lock:
+        _thumb_path(thumb_id).write_bytes(data)
+
+
+def get_thumbnail(thumb_id: str) -> bytes | None:
+    """Get thumbnail data."""
+    init()
+
+    thumb_file = _thumb_path(thumb_id)
+    if not thumb_file.exists():
+        return None
+
+    try:
+        return thumb_file.read_bytes()
+    except OSError:
+        return None
+
+
+def delete_thumbnail(thumb_id: str) -> None:
+    """Delete a thumbnail."""
+    init()
+
+    with _lock:
+        _thumb_path(thumb_id).unlink(missing_ok=True)
+
+
+def cleanup_expired(max_age_seconds: float) -> int:
+    """
+    Delete expired temp files.
+
+    Args:
+        max_age_seconds: Maximum age in seconds before expiry
+
+    Returns:
+        Number of files deleted
+    """
+    init()
+
+    now = time.time()
+    deleted = 0
+
+    with _lock:
+        # Find all metadata files
+        for meta_file in _temp_dir.glob("*.json"):
+            try:
+                metadata = json.loads(meta_file.read_text())
+                timestamp = metadata.get("timestamp", 0)
+
+                if now - timestamp > max_age_seconds:
+                    file_id = meta_file.stem
+                    _data_path(file_id).unlink(missing_ok=True)
+                    meta_file.unlink(missing_ok=True)
+                    # Also delete thumbnail if exists
+                    _thumb_path(f"{file_id}_thumb").unlink(missing_ok=True)
+                    deleted += 1
+            except (OSError, json.JSONDecodeError):
+                # Remove corrupted files
+                meta_file.unlink(missing_ok=True)
+                deleted += 1
+
+    return deleted
+
+
+def cleanup_all() -> int:
+    """
+    Delete all temp files. Call on service start/stop.
+
+    Returns:
+        Number of files deleted
+    """
+    init()
+
+    deleted = 0
+
+    with _lock:
+        for f in _temp_dir.iterdir():
+            if f.is_file():
+                f.unlink(missing_ok=True)
+                deleted += 1
+
+    return deleted
+
+
+def get_stats() -> dict:
+    """Get temp storage statistics."""
+    init()
+
+    files = list(_temp_dir.glob("*.data"))
+    total_size = sum(f.stat().st_size for f in files if f.exists())
+
+    return {
+        "file_count": len(files),
+        "total_size_bytes": total_size,
+        "temp_dir": str(_temp_dir),
+    }