fix: Remove hardcoded ports, support custom port configuration

- Replace all hardcoded localhost:8000/3000/8501 with environment variables
- Frontend: Use API_PORT env var in next.config.ts
- Backend: Use API_PORT env var in editor.py and quality.py
- Web UI: Use API_PORT and EDITOR_PORT env vars in all Streamlit pages
- Update dev.sh to pass environment variables to all services
- Add .env.example with port configuration template

Now supports custom ports via environment variables:
  API_PORT=8080 EDITOR_PORT=3001 WEB_PORT=8502 ./dev.sh

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

web/components/output_preview.py

@@ -26,6 +26,10 @@ from web.utils.async_helpers import run_async
 from pixelle_video.models.progress import ProgressEvent
 from pixelle_video.config import config_manager
 
+# Get ports from environment
+API_PORT = os.getenv("API_PORT", "8000")
+EDITOR_PORT = os.getenv("EDITOR_PORT", "3000")
+
 
 def render_output_preview(pixelle_video, video_params):
     """Render output preview section (right column)"""
@@ -135,7 +139,7 @@ def render_single_output(pixelle_video, video_params):
                     
                     # Submit to async API
                     response = requests.post(
-                        "http://localhost:8000/api/video/generate/async",
+                        f"http://localhost:{API_PORT}/api/video/generate/async",
                         json=api_payload,
                         timeout=30
                     )
@@ -309,7 +313,7 @@ def render_single_output(pixelle_video, video_params):
                             pass
                         
                         if task_id:
-                            editor_url = f"http://localhost:3000/editor?storyboard_id={task_id}"
+                            editor_url = f"http://localhost:{EDITOR_PORT}/editor?storyboard_id={task_id}"
                             st.markdown(
                                 f'''
                                 <a href="{editor_url}" target="_blank" style="text-decoration: none;">