Refactor CLI and add preprocessing pipeline

- New CLI structure: -p project, -a action (required flags)
- Add -i import, -f force, -v verbose, --dry-run, --no-cache options
- Add preprocessor.py with chroma key filter (ProRes 4444 output)
- Support background images from shared_assets folder
- Support video metadata JSON files (talkinghead.json)
- Add validation for preprocessed output before render
- Update gnommo.sh with import command and new CLI interface
- Fix Python 3.9 compatibility (Optional[] instead of | None)

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

gnommo/validator.py

@@ -74,12 +74,28 @@ def validate_project(
                 project_path / "videos.json"
             ))
 
-    # Check background video exists (if specified)
-    if config.background_video:
-        bg_path = project_path / config.background_video
+        # Check preprocessed output exists if preprocessing is defined
+        if video_source.preprocess and video_source.output_file:
+            output_path = project_path / video_source.output_file
+            if not output_path.exists():
+                issues.append(ValidationIssue(
+                    f"Preprocessed output not found: {video_source.output_file}. "
+                    f"Run with -a preprocess first.",
+                    project_path / "videos.json"
+                ))
+
+    # Check background exists (image or video)
+    # Try 'background' first, fall back to deprecated 'background_video'
+    bg_file = config.background or config.background_video
+    if bg_file:
+        # Check in project folder first, then parent (for shared_assets)
+        bg_path = project_path / bg_file
+        if not bg_path.exists():
+            # Try parent directory (shared_assets at repo root)
+            bg_path = project_path.parent / bg_file
         if not bg_path.exists():
             issues.append(ValidationIssue(
-                f"Background video not found: {config.background_video}",
+                f"Background not found: {bg_file}",
                 project_path / "project.json"
             ))