feat(quotesdb): add build.rs to convert api/openapi.yaml to JSON at compile time

- Create build.rs at crate root using serde_yaml (build-dep only) to parse
  api/openapi.yaml and write compact JSON to $OUT_DIR/openapi.json
- cargo:rerun-if-changed ensures re-conversion only when spec changes
- serde_yaml never enters the Workers or native binary (build-dep only)
- Downstream GET /api/ handler consumes via include_str!(concat!(env!("OUT_DIR"), "/openapi.json"))

Closes ticket 8892d5

quotesdb/build.rs

@@ -0,0 +1,31 @@
+//! Build script for the `quotesdb` crate.
+//!
+//! Reads `api/openapi.yaml`, parses it into a `serde_json::Value`, and writes
+//! compact JSON to `$OUT_DIR/openapi.json` at compile time.
+//!
+//! The API handler uses the output via:
+//! ```ignore
+//! const OPENAPI_JSON: &str = include_str!(concat!(env!("OUT_DIR"), "/openapi.json"));
+//! ```
+//!
+//! Using `serde_yaml` as a build-dependency only means it never enters the
+//! Workers binary or native binary — zero runtime cost. The conversion runs
+//! only when `api/openapi.yaml` changes, thanks to `cargo:rerun-if-changed`.
+
+use std::{env, fs, path::Path};
+
+fn main() {
+    // Re-run this script whenever the OpenAPI spec changes.
+    println!("cargo:rerun-if-changed=api/openapi.yaml");
+
+    let yaml = fs::read_to_string("api/openapi.yaml").expect("failed to read api/openapi.yaml");
+
+    let value: serde_json::Value =
+        serde_yaml::from_str(&yaml).expect("failed to parse api/openapi.yaml");
+
+    let json = serde_json::to_string(&value).expect("JSON serialisation failed");
+
+    let out_dir = env::var("OUT_DIR").expect("OUT_DIR not set");
+    let out_path = Path::new(&out_dir).join("openapi.json");
+    fs::write(&out_path, json).expect("failed to write openapi.json");
+}