fix: Auction end_time timezone - add UTC suffix

The frontend was calculating wrong time remaining because
end_time was sent without timezone suffix (Z).

JavaScript's new Date() interprets "2025-12-20T20:49:20" as
local time, but the server stores it as UTC.

Fix: Add json_encoders to Pydantic models that append "Z"
to all datetime fields, marking them as UTC.

Affected models:
- AuctionListing
- AuctionSearchResponse
- MarketFeedItem
- MarketFeedResponse
- ScrapeStatus

backend/app/api/auctions.py

@@ -78,6 +78,10 @@ class AuctionListing(BaseModel):
     
     class Config:
         from_attributes = True
+        # Serialize datetimes as ISO format with UTC timezone suffix
+        json_encoders = {
+            datetime: lambda v: v.isoformat() + "Z" if v else None
+        }
 
 
 class AuctionSearchResponse(BaseModel):
@@ -92,6 +96,11 @@ class AuctionSearchResponse(BaseModel):
         "$50 × Length × TLD × Keyword × Brand factors. "
         "See /portfolio/valuation/{domain} for detailed breakdown."
     )
+    
+    class Config:
+        json_encoders = {
+            datetime: lambda v: v.isoformat() + "Z" if v else None
+        }
 
 
 class PlatformStats(BaseModel):
@@ -108,6 +117,11 @@ class ScrapeStatus(BaseModel):
     total_auctions: int
     platforms: List[str]
     next_scrape: Optional[datetime]
+    
+    class Config:
+        json_encoders = {
+            datetime: lambda v: v.isoformat() + "Z" if v else None
+        }
 
 
 class MarketFeedItem(BaseModel):
@@ -146,6 +160,9 @@ class MarketFeedItem(BaseModel):
     
     class Config:
         from_attributes = True
+        json_encoders = {
+            datetime: lambda v: v.isoformat() + "Z" if v else None
+        }
 
 
 class MarketFeedResponse(BaseModel):
@@ -157,6 +174,11 @@ class MarketFeedResponse(BaseModel):
     sources: List[str]
     last_updated: datetime
     filters_applied: dict = {}
+    
+    class Config:
+        json_encoders = {
+            datetime: lambda v: v.isoformat() + "Z" if v else None
+        }
 
 
 # ============== Helper Functions ==============