feat(backend): /settings GET+PUT, /models PiModel shape, app wiring

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/routes/settings.ts

@@ -0,0 +1,48 @@
+import type { FastifyInstance } from "fastify";
+import type { AppConfig } from "../config.js";
+import { loadSettings, saveSettings, type Settings } from "../settings/settings-store.js";
+import { listWorkspaces, type ListModelsFn } from "./meta.js";
+
+/** Merge stored settings over env/first-workspace defaults. */
+export function effectiveSettings(cfg: AppConfig, stored: Settings): Settings {
+  const workspaces = listWorkspaces(cfg.harnessDir);
+  return {
+    workspace: stored.workspace ?? (workspaces[0]?.name),
+    provider: stored.provider ?? cfg.defaults.provider,
+    model: stored.model ?? cfg.defaults.model,
+    thinking: stored.thinking ?? cfg.defaults.thinking,
+  };
+}
+
+export function settingsRoutes(
+  app: FastifyInstance,
+  deps: { cfg: AppConfig; listModels: ListModelsFn },
+): void {
+  app.get("/settings", async () => {
+    return effectiveSettings(deps.cfg, loadSettings(deps.cfg));
+  });
+
+  app.put("/settings", async (req, reply) => {
+    const b = (req.body ?? {}) as Settings;
+    if (b.model) {
+      let available: { id: string }[] = [];
+      try {
+        available = await deps.listModels();
+      } catch {
+        available = [];
+      }
+      // Only validate when Pi gave us a non-empty list; otherwise allow (degraded).
+      if (available.length > 0 && !available.some((m) => m.id === b.model)) {
+        return reply.code(400).send({ error: `Unknown model: ${b.model}` });
+      }
+    }
+    const next: Settings = {
+      workspace: b.workspace,
+      provider: b.provider,
+      model: b.model,
+      thinking: b.thinking,
+    };
+    saveSettings(deps.cfg, next);
+    return effectiveSettings(deps.cfg, next);
+  });
+}