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>

api/routers/editor.py

@@ -47,24 +47,31 @@ from api.schemas.editor import (
 from fastapi import BackgroundTasks
 import asyncio
 import uuid as uuid_module
+import os
 
 router = APIRouter(prefix="/editor", tags=["Editor"])
 
 # Export task storage
 _export_tasks: dict = {}
 
+# Get API port from environment
+API_PORT = os.getenv("API_PORT", "8000")
 
-def _path_to_url(file_path: str, base_url: str = "http://localhost:8000") -> str:
+
+def _path_to_url(file_path: str, base_url: str = None) -> str:
     """Convert local file path to URL accessible through API"""
     if not file_path:
         return None
-    
+
+    if base_url is None:
+        base_url = f"http://localhost:{API_PORT}"
+
     import os
     from pathlib import Path
-    
+
     # Normalize path separators
     file_path = file_path.replace("\\", "/")
-    
+
     # Extract relative path from output directory
     parts = file_path.split("/")
     try:
@@ -73,7 +80,7 @@ def _path_to_url(file_path: str, base_url: str = "http://localhost:8000") -> str
         relative_path = "/".join(relative_parts)
     except ValueError:
         relative_path = Path(file_path).name
-    
+
     return f"{base_url}/api/files/{relative_path}"
 
 
@@ -951,8 +958,9 @@ async def export_video(
             for frame in sorted_frames:
                 path = frame.get("video_segment_path", "")
                 if path.startswith("http"):
-                    # Extract path from URL
-                    path = path.replace("http://localhost:8000/api/files/", "output/")
+                    # Extract path from URL (format: http://localhost:{port}/api/files/{relative_path})
+                    if "/api/files/" in path:
+                        path = "output/" + path.split("/api/files/")[-1]
                 video_segments.append(path)
             
             _export_tasks[task_id]["progress"] = 0.3

api/routers/quality.py

@@ -349,8 +349,9 @@ async def extract_style(
         # Convert URL to file path if needed
         actual_path = image_path
         if image_path.startswith("http"):
-            # Extract path from URL like http://localhost:8000/api/files/...
-            actual_path = image_path.replace("http://localhost:8000/api/files/", "output/")
+            # Extract path from URL (format: http://localhost:{port}/api/files/{relative_path})
+            if "/api/files/" in image_path:
+                actual_path = "output/" + image_path.split("/api/files/")[-1]
         
         # Check if file exists
         import os