Declare typed Convex env and route auth through @code/env/convex

- Register SITE_URL, NATIVE_APP_URL, CONVEX_SITE_URL, FLUE_DB_TOKEN in
  convex.config.ts so the generated server.js exposes a typed env object.
- Rewrite auth.ts to consume @code/env/convex and tighten the
  getCurrentUser handler.
- Refresh the backend README to describe the control-plane role and
  generated-file policy.

packages/backend/convex/_generated/api.d.ts

@@ -12,6 +12,7 @@ import type * as auth from "../auth.js";
 import type * as daemonCommands from "../daemonCommands.js";
 import type * as daemonRuntime from "../daemonRuntime.js";
 import type * as daemons from "../daemons.js";
+import type * as fluePersistence from "../fluePersistence.js";
 import type * as healthCheck from "../healthCheck.js";
 import type * as http from "../http.js";
 import type * as privateData from "../privateData.js";
@@ -28,6 +29,7 @@ declare const fullApi: ApiFromModules<{
   daemonCommands: typeof daemonCommands;
   daemonRuntime: typeof daemonRuntime;
   daemons: typeof daemons;
+  fluePersistence: typeof fluePersistence;
   healthCheck: typeof healthCheck;
   http: typeof http;
   privateData: typeof privateData;

packages/backend/convex/_generated/server.d.ts

@@ -21,6 +21,15 @@ import {
 } from "convex/server";
 import type { DataModel } from "./dataModel.js";
 
+/**
+ * Typesafe environment variables declared in `convex.config.ts`.
+ */
+type Env = {
+  readonly FLUE_DB_TOKEN: string;
+  readonly NATIVE_APP_URL: string | undefined;
+  readonly SITE_URL: string;
+};
+
 /**
  * Define a query in this Convex app's public API.
  *
@@ -95,6 +104,11 @@ export declare const internalAction: ActionBuilder<DataModel, "internal">;
  */
 export declare const httpAction: HttpActionBuilder;
 
+/**
+ * Typesafe environment variables declared in `convex.config.ts`.
+ */
+export declare const env: Env;
+
 /**
  * A set of services for use within Convex query functions.
  *

packages/backend/convex/_generated/server.js

@@ -91,3 +91,8 @@ export const internalAction = internalActionGeneric;
  * @returns The wrapped function. Import this function from `convex/http.js` and route it to hook it up.
  */
 export const httpAction = httpActionGeneric;
+
+/**
+ * Typesafe environment variables declared in `convex.config.ts`.
+ */
+export const env = process.env;