Index backend config fields cannot be resolved from environment variables or secrets

### Summary Index backend configuration fields (e.g. RocksDB `path`, Redis `connectionString`) could not be resolved from environment variables or secrets, unlike other plugin config values (Sources, Reactions, etc.) which support `${ENV}` and secret resolution via the shared `ConfigValue` mechanism. ### Why this is a problem - **No parity with other plugins.** Connection strings frequently contain credentials and should be injectable via secrets; storage paths often need to be environment-specific. Hard-coding them in config (or only supplying them via injected Rust code) is inconsistent and insecure. - **Blocks declarative deployment.** Without env/secret resolution, the same config cannot be promoted across environments. ### Expected behavior Index backend config fields should resolve from static values, `${ENV}` variables, and secrets — and be validated *after* resolution — consistent with how other plugin DTOs work. ### Affected code - `components/indexes/rocksdb` - `components/indexes/garnet` Addressed by #496.