templates: one Jinja language (hive-template) for the notify router's flows, flows document version 2

A todo node's key and text were hand-expanded `{field}` strings. They are now Jinja templates rendered through the new hive-template crate (minijinja, an unknown variable is an error), and the controller's save check reads the variables the template uses. The flows document is version 2; a version 1 document reads as version 2 with each `{field}` converted to `{{ field }}`, and the next save writes it.

hive-template/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "hive-template"
+edition.workspace = true
+version.workspace = true
+readme = "README.md"
+
+[lints]
+workspace = true
+
+[dependencies]
+minijinja = { version = "2", default-features = false, features = [
+  "builtins",
+  "serde",
+] }
+serde.workspace = true
+
+[dev-dependencies]
+serde_json.workspace = true