feat(network): thumbnail generation on save, shown on list page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/network_diagrams.py

@@ -1,10 +1,12 @@
 """Network diagrams API endpoints."""
+import base64
 import logging
 from datetime import datetime, timezone
 from typing import Annotated
 from uuid import UUID
 
 from fastapi import APIRouter, Depends, HTTPException, Query
+from pydantic import BaseModel
 from sqlalchemy import select, or_
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -27,7 +29,7 @@ from app.schemas.network_diagram import (
     DiagramNode,
     DiagramEdge,
 )
-from app.services import network_diagram_ai_service
+from app.services import network_diagram_ai_service, storage_service
 
 # Maps system device-type slugs to their category — mirrors frontend deviceRegistry.ts
 _SLUG_CATEGORY: dict[str, str] = {
@@ -83,6 +85,7 @@ def _diagram_to_list_item(
         description=diagram.description,
         node_count=len(nodes),
         category_counts=category_counts,
+        thumbnail_url=diagram.thumbnail_url,
         created_by=diagram.created_by,
         created_at=diagram.created_at,
         updated_at=diagram.updated_at,
@@ -305,6 +308,34 @@ async def import_diagram(
     )
 
 
+class ThumbnailUploadRequest(BaseModel):
+    data_url: str  # base64 PNG data URL: "data:image/png;base64,..."
+
+
+@router.post("/{diagram_id}/thumbnail", status_code=204)
+async def upload_thumbnail(
+    diagram_id: UUID,
+    body: ThumbnailUploadRequest,
+    db: Annotated[AsyncSession, Depends(get_db)],
+    current_user: Annotated[User, Depends(get_current_active_user)],
+) -> None:
+    diagram = await _get_diagram_or_404(diagram_id, current_user.account_id, db)
+    try:
+        header, encoded = body.data_url.split(",", 1)
+    except ValueError:
+        raise HTTPException(status_code=422, detail="Invalid data URL format")
+    image_bytes = base64.b64decode(encoded)
+    storage_key = await storage_service.upload_file(
+        file_data=image_bytes,
+        filename=f"thumbnail-{diagram_id}.png",
+        content_type="image/png",
+        account_id=str(current_user.account_id),
+    )
+    presigned_url = storage_service.get_presigned_url(storage_key)
+    diagram.thumbnail_url = presigned_url
+    await db.commit()
+
+
 @router.post("/ai-generate", response_model=AIGenerateResponse)
 async def ai_generate_diagram(
     data: AIGenerateRequest,

backend/app/schemas/network_diagram.py

@@ -92,6 +92,7 @@ class NetworkDiagramListItem(BaseModel):
     description: str | None = None
     node_count: int = 0
     category_counts: dict[str, int] = Field(default_factory=dict)
+    thumbnail_url: str | None = None
     created_by: UUID | None = None
     created_at: datetime
     updated_at: datetime