feat: read-only live view at /view?key=<LIVEVIEW_KEY>

Implements issue #5. Off by default; set LIVEVIEW_KEY in .env to enable.
No JWT required — key-based auth via ?key= query param.
Returns 403 when disabled or key is wrong.
Read-only ReactFlow canvas (pan/zoom, no editing).
Standalone mode loads from localStorage without a key.
Includes 8 backend tests and 9 frontend tests.

backend/app/api/routes/liveview.py

@@ -0,0 +1,40 @@
+from typing import Any
+
+from fastapi import APIRouter, Depends, HTTPException, Query
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.config import settings
+from app.db.database import get_db
+from app.db.models import CanvasState, Edge, Node
+from app.schemas.canvas import CanvasStateResponse
+from app.schemas.edges import EdgeResponse
+from app.schemas.nodes import NodeResponse
+
+router = APIRouter()
+
+
+@router.get("", response_model=CanvasStateResponse)
+async def liveview_canvas(
+    key: str | None = Query(default=None),
+    db: AsyncSession = Depends(get_db),
+) -> CanvasStateResponse:
+    """Read-only public canvas endpoint.
+
+    Disabled by default — requires LIVEVIEW_KEY to be set in .env.
+    Always returns 403 when disabled, regardless of the key provided.
+    """
+    if not settings.liveview_key:
+        raise HTTPException(status_code=403, detail="Live view is disabled")
+    if not key or key != settings.liveview_key:
+        raise HTTPException(status_code=403, detail="Invalid live view key")
+
+    nodes = (await db.execute(select(Node))).scalars().all()
+    edges = (await db.execute(select(Edge))).scalars().all()
+    state = await db.get(CanvasState, 1)
+    viewport: dict[str, Any] = state.viewport if state else {"x": 0, "y": 0, "zoom": 1}
+    return CanvasStateResponse(
+        nodes=[NodeResponse.model_validate(n) for n in nodes],
+        edges=[EdgeResponse.model_validate(e) for e in edges],
+        viewport=viewport,
+    )