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.

.env.example

@@ -22,3 +22,8 @@ STATUS_CHECKER_INTERVAL=60
 # Generate keys: python3 -c "import secrets; print(secrets.token_hex(32))"
 MCP_API_KEY=mcp_sk_changeme
 MCP_SERVICE_KEY=svc_changeme
+
+# Live view — read-only public canvas at /view?key=<value>
+# Off by default. Set to a random secret to enable.
+# Generate: python3 -c "import secrets; print(secrets.token_urlsafe(32))"
+# LIVEVIEW_KEY=