Implement job deletion credit refunds

Added proper refund logic using CreditTransactionManager:
- Full refund if job never started (no third_party_id)
- Partial refund for queued jobs (reserves - 2 credits penalty)
- No refund for processing/completed jobs
- Full refund for failed jobs

All refunds properly tracked in credit_transactions table
with metadata about job type and status.

Resolves TODO at line 564.

routers/gemini.py

@@ -561,11 +561,62 @@ async def delete_job(
             detail="Job not found"
         )
     
-    # TODO: Implement credit refund via CreditTransactionManager if needed
-    # For now, just mark job as deleted
+    # Implement credit refund via CreditTransactionManager
+    refund_amount = 0
     message = "Job deleted"
     
-    # Soft delete the job using DeleteQuery
+    # Determine refund based on job status
+    if job.credits_reserved > 0 and not job.credits_refunded:
+        from services.credit_service import CreditTransactionManager
+        
+        if not job.third_party_id:
+            # Job never started (pre-execution failure)
+            refund_amount = job.credits_reserved
+            refund_reason = "Job deleted before execution"
+            
+        elif job.status == "queued":
+            # Job queued - partial refund (2 credits cancellation fee)
+            penalty = 2
+            refund_amount = max(0, job.credits_reserved - penalty)
+            refund_reason = f"Job cancelled while queued (penalty: {penalty} credits)"
+            
+        elif job.status in ["processing", "completed"]:
+            # No refund for started/completed jobs
+            refund_amount = 0
+            refund_reason = None
+            
+        elif job.status == "failed":
+            # Failed job - full refund
+            refund_amount = job.credits_reserved
+            refund_reason = "Job failed - full refund"
+        
+        # Process refund if applicable
+        if refund_amount > 0:
+            try:
+                await CreditTransactionManager.add_credits(
+                    session=db,
+                    user=user,
+                    amount=refund_amount,
+                    source="job_deletion",
+                    reference_type="job",
+                    reference_id=job.job_id,
+                    reason=refund_reason,
+                    metadata={
+                        "job_type": job.job_type,
+                        "original_cost": job.credits_reserved,
+                        "job_status": job.status
+                    }
+                )
+                
+                job.credits_refunded = True
+                await db.commit()
+                message = f"Job deleted. {refund_amount} credits refunded."
+                
+            except Exception as e:
+                logger.error(f"Failed to refund credits for job {job_id}: {e}")
+                message = "Job deleted (refund failed - contact support)"
+    
+    # Soft delete the job
     deleted = await qs.delete().soft_delete_one(job)
     
     if not deleted:
@@ -577,7 +628,7 @@ async def delete_job(
     return {
         "success": True,
         "message": message,
-        "refund_amount": 0,  # Refunds handled by transaction manager if needed
+        "refund_amount": refund_amount,
         "new_credit_balance": user.credits
     }