Return existing asset instead of error when asset already exists

When publishing to L2, if the asset already exists, return it with
existing: true flag instead of raising a 400 error. This makes
re-publishing idempotent and handles race conditions gracefully.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server.py

@@ -1784,9 +1784,11 @@ async def _register_asset_impl(req: RegisterRequest, owner: str):
     if not req.ipfs_cid:
         raise HTTPException(400, "IPFS CID is required for registration")
 
-    # Check if name exists
-    if await db.asset_exists(req.name):
-        raise HTTPException(400, f"Asset already exists: {req.name}")
+    # Check if name exists - return existing asset if so
+    existing = await db.get_asset_by_name(req.name)
+    if existing:
+        logger.info(f"register_asset: Asset {req.name} already exists, returning existing")
+        return {"asset": existing, "activity": None, "existing": True}
 
     # ===== PHASE 2: IPFS OPERATIONS (non-blocking) =====
     import asyncio
@@ -1805,7 +1807,10 @@ async def _register_asset_impl(req: RegisterRequest, owner: str):
         async with db.transaction() as conn:
             # Check name again inside transaction (race condition protection)
             if await db.asset_exists_by_name_tx(conn, req.name):
-                raise HTTPException(400, f"Asset already exists: {req.name}")
+                # Race condition - another request created it first, return existing
+                existing = await db.get_asset_by_name(req.name)
+                logger.info(f"register_asset: Asset {req.name} created by concurrent request")
+                return {"asset": existing, "activity": None, "existing": True}
 
             # Create asset
             asset = {
@@ -2084,9 +2089,11 @@ async def record_run(req: RecordRunRequest, user: User = Depends(get_required_us
                     }
                     await db.create_asset_tx(conn, input_asset)
 
-            # Check output doesn't already exist (by content_hash)
+            # Check if output already exists (by content_hash) - return existing if so
             if await db.asset_exists_by_name_tx(conn, output_hash):
-                raise HTTPException(400, f"Asset already exists: {output_hash[:16]}...")
+                existing = await db.get_asset_by_name(output_hash)
+                logger.info(f"record_run: Output {output_hash[:16]}... already exists")
+                return {"asset": existing, "activity": None, "existing": True}
 
             # Create output asset with provenance - named by content_hash
             output_asset = {