Generate ampd config schema via schemars (schema-rs) and wire into codegen

[LNSD]

Goal: Generate a JSON schema for the ampd config file using schemars (schema-rs) and integrate it into the
existing codegen workflow.\n\nContext:\n- The current codegen flow is driven by the justfile gen target, which runs
cargo check --workspace with RUSTFLAGS=\\\"--cfg gen_schema_manifest --cfg gen_schema_tables --cfg gen_openapi_spec\ \\" and then copies artifacts from target/debug/build/*/out/ into docs.\n\nScope:\n- Add schema-rs generation for
the ampd config type(s) (using schemars).\n- Emit schema artifacts to an appropriate output directory under docs (align
with existing schema output structure).\n- Wire the new schema generation into just gen or a new codegen target,
consistent with the current pattern of using RUSTFLAGS + build scripts that emit to target/debug/build/*/out/.\n-
Update docs/examples to reference the generated config schema.