fix: require projectUrl for classify_st_url endpoint

[vee1e]

Summary

The /api/classify_st_url endpoint was returning HTTP 200 with an empty predictions array when no projectUrl was provided, which is confusing to API consumers.

Before (Issue)

When making a request without a projectUrl:

Request:

POST /api/classify_st_url
Content-Type: application/json

{
  "projectName": "Test Project",
  "projectDescription": "A test description"
}

Response (HTTP 200 OK):

{
  "projectName": "Test Project",
  "projectUrl": null,
  "predictions": []
}

Problems:

• HTTP 200 suggests success, but no predictions were made
• Empty predictions array with no explanation why
• API consumers might think this is a bug
• No clear indication that URL is required for URL-based classification

After (Fixed)

Same request now returns:

Response (HTTP 400 Bad Request):

{
  "error": "Project URL is required for URL-based classification"
}

Benefits:

• Clear error message tells user exactly what is needed
• HTTP 400 indicates client error (missing required field)
• No confusing empty predictions array
• Aligns with endpoint name (URL-based classification)

Changes Made

In backend/app.py, the classify_st_url function:

• Changed from conditional if/else block to early validation
• Removed the else branch that returned a message-only response
• Now returns 400 Bad Request immediately when projectUrl is missing
• Simplified control flow and removed dead code

The fix is minimal and focused on improving API usability through proper error handling.