API: versioned endpoint listing understood RIAPI keys per backend

[lilith]

Summary

Add a versioned JSON API endpoint that returns all understood RIAPI querystring keys, tagged by backend (v2 and zen).

Currently v1/schema/riapi/latest/list_keys returns a flat list of 100 key names from IR4_KEYS with no backend distinction, no type metadata, and no schema information.

Proposed endpoints

v1/schema/riapi/v1/keys

Returns understood keys per backend:

• v2: The 100 keys from IR4_KEYS in imageflow_riapi::ir4::parsing
• zen: Keys declared via #[kv(...)] annotations on zennode structs across zenlayout, zenresize, zenfilters, and zencodecs

Response includes backend name, version identifier, sorted key list, and count.

v1/schema/riapi/v1/zen-node-schema

Returns the full zennode JSON Schema (2020-12) for the zen RIAPI node registry. Uses zennode::json_schema::registry_to_json_schema(). Includes types, ranges, defaults, descriptions, and x-zennode-kv-keys extensions mapping RIAPI keys to node parameters.

Only available when zen-pipeline feature is enabled.

Implementation notes

• Extract build_registry() helper in imageflow_core/src/zen/riapi.rs
• Add json-schema feature to zennode dependency
• Zen endpoints gated behind #[cfg(feature = "zen-pipeline")]
• Add response types to imageflow_types