[WIP] fix: job_description cannot support null value

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Original issue description:

Bug Report

Describe the bug

A clear and concise description of what the bug is. For example: "AI job description extraction fails when processing job postings due to database constraint violation on employment_type field."

To Reproduce

Steps to reproduce the behavior:

1. Navigate to the AI extraction management page
2. Select job descriptions to process with AI extraction
3. Click on 'Start Extraction' or run the extraction command
4. See error: IntegrityError: NOT NULL constraint failed: application_jobdescription.employment_type

Expected behavior

The AI extraction should successfully process job descriptions and create JobDescription records in the database, even when some fields like employment_type are not detected or are null in the extracted data.

Environment Information:

Backend (Django)

• OS: macOS
• Python Version: [e.g. 3.11]
• Django Version: [e.g. 4.2]
• Database: [e.g. SQLite/PostgreSQL]

Frontend (if applicable)

• OS: macOS
• Browser: [e.g. Chrome, Safari, Firefox]
• Version: [e.g. 116]

AI Services

• OpenAI API Version: [e.g. gpt-4]
• LangChain Version: [e.g. 0.0.340]

Error Details

Traceback (most recent call last):
  File "/path/to/applyday/backend/applyday/ai/services/extract_jd.py", line X, in process_extract
    jd = JobDescription.objects.create(job_text=job, **data)
django.db.utils.IntegrityError: NOT NULL constraint failed: application_jobdescription.employment_type

Affected Components

• AI Job Description Extraction (ai/services/extract_jd.py)
• JobDescription Model (application/models.py)
• Database Schema
• Admin Interface
• API Endpoints

Severity

• Critical - System unusable
• High - Major functionality broken
• Medium - Some functionality affected
• Low - Minor issue

Additional context

• This issue occurs when the AI extraction service returns None or missing values for required database fields
• The JobDescription model has non-nullable fields that conflict with incomplete AI extraction results
• Related to the database migration and model constraints setup
• May affect batch processing of multiple job descriptions

Proposed Solution

1. Modify JobDescription model to allow null values for AI-extracted fields
2. Add default values in the extraction service for required fields
3. Create and apply database migration to update constraints
4. Add validation and error handling for incomplete extractions

Related Files

• backend/applyday/application/models.py
• backend/applyday/ai/services/extract_jd.py
• backend/applyday/ai/schemas/job_description.py

Fixes #31.

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