fix: Sync News API models with actual MongoDB schema

## 🔧 Model Synchronization
Updated Pydantic models to match actual article structure in MongoDB

### Changes
- **Article Model**: Complete restructure to match MongoDB documents
  - Added Subtopic, Reference, Entities nested models
  - Changed created_at to Union[str, datetime] with serializer
  - Added all pipeline metadata fields (job_id, keyword_id, etc.)
  - Added translation & image fields
  - Changed category (single) to categories (array)

- **ArticleSummary Model**: Updated for list responses
  - Synced with actual MongoDB structure
  - Added news_id, categories array, images array

- **ArticleService**: Fixed category filtering
  - Changed "category" to "categories" (array field)
  - Updated search to include subtopics and source_keyword
  - Implemented MongoDB aggregation for category list

### Verified Fields
✅ news_id, title, summary, created_at, language
✅ subtopics (array of {title, content[]})
✅ categories (array), entities (nested object)
✅ references (array), source_keyword, source_count
✅ pipeline_stages, job_id, keyword_id, processing_time
✅ images (array), image_prompt, translated_languages

### Testing
- Validated with actual English articles (20,966 total)
- Search functionality working (15,298 AI-related articles)
- Categories endpoint returning 1000+ unique categories
- All datetime fields properly serialized to ISO format

🤖 Generated with [Claude Code](https://claude.ai/claude-code)

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

services/news-api/backend/app/models/article.py

@@ -1,34 +1,79 @@
-from pydantic import BaseModel, Field
-from typing import Optional, List
+from pydantic import BaseModel, Field, field_serializer
+from typing import Optional, List, Dict, Any, Union
 from datetime import datetime
 
+class Subtopic(BaseModel):
+    title: str
+    content: List[str]
+
+class Reference(BaseModel):
+    title: str
+    link: str
+    source: str
+    published: Optional[str] = None
+
+class Entities(BaseModel):
+    people: List[str] = []
+    organizations: List[str] = []
+    groups: List[str] = []
+    countries: List[str] = []
+    events: List[str] = []
+
 class Article(BaseModel):
     id: str = Field(alias="_id")
+    news_id: str
     title: str
-    content: str
     summary: Optional[str] = None
+    created_at: Union[str, datetime]
     language: str
-    category: Optional[str] = None
-    tags: Optional[List[str]] = []
-    source_url: Optional[str] = None
-    image_url: Optional[str] = None
-    author: Optional[str] = None
-    published_at: Optional[datetime] = None
-    created_at: datetime
-    updated_at: Optional[datetime] = None
+
+    @field_serializer('created_at')
+    def serialize_created_at(self, value: Union[str, datetime], _info):
+        if isinstance(value, datetime):
+            return value.isoformat()
+        return value
+
+    # Content fields
+    subtopics: List[Subtopic] = []
+    categories: List[str] = []
+    entities: Optional[Entities] = None
+
+    # Source information
+    source_keyword: Optional[str] = None
+    source_count: Optional[int] = None
+    references: List[Reference] = []
+
+    # Pipeline metadata
+    job_id: Optional[str] = None
+    keyword_id: Optional[str] = None
+    pipeline_stages: List[str] = []
+    processing_time: Optional[float] = None
+
+    # Translation & Image
+    ref_news_id: Optional[str] = None
+    rss_guid: Optional[str] = None
+    image_prompt: Optional[str] = None
+    images: List[str] = []
+    translated_languages: List[str] = []
 
     class Config:
         populate_by_name = True
         json_schema_extra = {
             "example": {
                 "_id": "507f1f77bcf86cd799439011",
+                "news_id": "uuid-string",
                 "title": "Sample News Article",
-                "content": "This is the full content of the article...",
-                "summary": "A brief summary of the article",
-                "language": "ko",
-                "category": "technology",
-                "tags": ["AI", "tech", "innovation"],
-                "created_at": "2024-01-01T00:00:00Z"
+                "summary": "A brief summary",
+                "language": "en",
+                "created_at": "2024-01-01T00:00:00Z",
+                "subtopics": [
+                    {
+                        "title": "Main Topic",
+                        "content": ["Content paragraph 1", "Content paragraph 2"]
+                    }
+                ],
+                "categories": ["technology", "business"],
+                "images": ["http://image-url.com/image.png"]
             }
         }
 
@@ -41,13 +86,20 @@ class ArticleList(BaseModel):
 
 class ArticleSummary(BaseModel):
     id: str = Field(alias="_id")
+    news_id: str
     title: str
     summary: Optional[str] = None
     language: str
-    category: Optional[str] = None
-    image_url: Optional[str] = None
-    published_at: Optional[datetime] = None
-    created_at: datetime
+    categories: List[str] = []
+    images: List[str] = []
+    created_at: Union[str, datetime]
+    source_keyword: Optional[str] = None
+
+    @field_serializer('created_at')
+    def serialize_created_at(self, value: Union[str, datetime], _info):
+        if isinstance(value, datetime):
+            return value.isoformat()
+        return value
 
     class Config:
         populate_by_name = True