bugfix: proxy /api/* through the SvelteKit container

The compose deploy was unreachable because frontend code reads its
API base from `import.meta.env.VITE_API_BASE` at build time, but the
shipped image baked in the fallback `/api` and never picked up the
`PUBLIC_API_BASE` env var. The browser then hit
http://localhost:3000/api/...which the Node adapter doesn't serve, so
every request 404'd.

Fix the topology at the right layer: hooks.server.ts proxies /api/*
requests through to the backend container over docker's internal
network. The browser only ever talks to :3000, cookies stay
same-origin, and CORS can stay empty.

- frontend/src/hooks.server.ts: new proxy. Reads BACKEND_URL (defaults
  to http://localhost:8080 for ad-hoc node builds). Strips `host` and
  `content-length` so the backend sees the real client request and
  recomputes the length. Sets `duplex: 'half'` for streamed POST
  bodies. GET/HEAD have no body. Non-/api paths fall through to
  SvelteKit normally.
- docker-compose.yml: drop the host port mapping on the backend
  (browser doesn't reach it directly anymore — use `ports:` instead of
  `expose:` if you want curl access). Set BACKEND_URL=http://backend:8080
  on the frontend service. Drop PUBLIC_API_BASE which was unused.
- .env.example: replace PUBLIC_API_BASE with BACKEND_URL, with a note
  on what it does.
- README: explain the new topology in Quick start, update the bot
  curl examples to hit :3000 (since that's the only published port in
  the default deploy), and call out that the TLS terminator only needs
  one upstream now.

Lockstep version bump to 0.9.1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -16,11 +16,14 @@ cp .env.example .env
 docker compose up --build
 ```
 
-| Service           | URL                                |
-| ----------------- | ---------------------------------- |
-| Frontend          | <http://localhost:3000>            |
-| API base          | <http://localhost:8080/api/v1>     |
-| Health check      | <http://localhost:8080/api/v1/health> |
+| Service                    | URL                                                                |
+| -------------------------- | ------------------------------------------------------------------ |
+| Frontend (and API browser) | <http://localhost:3000>                                            |
+| API health                 | <http://localhost:3000/api/v1/health>                              |
+
+The browser only ever talks to the frontend container on `:3000`. SvelteKit's [`hooks.server.ts`](frontend/src/hooks.server.ts) reverse-proxies `/api/*` to the backend service over docker's internal network, so cookies stay same-origin and you don't need to publish the backend port or configure CORS to get a working deploy.
+
+If you want to hit the backend directly (bot scripts, ops debugging), publish its port by editing the backend service in [docker-compose.yml](docker-compose.yml) — change `expose` to `ports: ["8080:8080"]`.
 
 The first boot runs the migrations automatically. From there:
 
@@ -142,21 +145,23 @@ The frontend handles this for you: register → cookie set → writes work. Cook
 
 ### Bots / scripts (bearer tokens)
 
+The frontend on `:3000` proxies `/api/*` through to the backend, so any URL below works against `http://localhost:3000` in the default compose deploy. If you publish the backend port directly, swap in `http://localhost:8080`.
+
 ```bash
 # 1. Log in once via cookies (or register).
-curl -sb -c cookies.txt -X POST http://localhost:8080/api/v1/auth/login \
+curl -sb -c cookies.txt -X POST http://localhost:3000/api/v1/auth/login \
   -H 'content-type: application/json' \
   -d '{"username":"alice","password":"hunter2hunter2"}'
 
 # 2. Mint a long-lived bot token. The `bearer` value is shown ONCE.
-curl -sb cookies.txt -X POST http://localhost:8080/api/v1/auth/tokens \
+curl -sb cookies.txt -X POST http://localhost:3000/api/v1/auth/tokens \
   -H 'content-type: application/json' \
   -d '{"name":"ci-bot"}'
 # → { "id": "...", "name": "ci-bot", "bearer": "raw-token-here", ... }
 
 # 3. Use the bearer from anywhere.
 curl -H 'Authorization: Bearer raw-token-here' \
-     http://localhost:8080/api/v1/auth/me
+     http://localhost:3000/api/v1/auth/me
 ```
 
 Tokens are stored hashed (sha256) at rest; the raw value never leaves the response that created it. Revoke with `DELETE /api/v1/auth/tokens/{id}` while authenticated as the owner.
@@ -177,13 +182,13 @@ All variables can be set in `.env` (for `docker compose`) or your shell (for `ca
 | `CORS_ALLOWED_ORIGINS`  | (empty → same-origin)    | Comma-separated origin allowlist.             |
 | `MAX_REQUEST_BYTES`     | `209715200` (200 MiB)    | Hard cap on multipart request size.           |
 | `MAX_FILE_BYTES`        | `20971520` (20 MiB)      | Cap on a single image part.                   |
-| `PUBLIC_API_BASE`       | `http://localhost:8080/api` | Browser-facing API base.                   |
+| `BACKEND_URL`           | `http://backend:8080`    | Where the frontend's `/api/*` proxy points.   |
 
 ## Deployment
 
 For real hosts:
 
-- **Front Mangalord with a TLS terminator** (Caddy, nginx, traefik). Point `:443` at the frontend on `:3000` and proxy `/api/*` to the backend on `:8080`. With same-origin routing you can leave `CORS_ALLOWED_ORIGINS` empty and the session cookie's `SameSite=Lax` does its job.
+- **Front Mangalord with a TLS terminator** (Caddy, nginx, traefik). Point `:443` at the frontend container on `:3000` — the SvelteKit proxy handles `/api/*` internally, so you only need one upstream. With same-origin routing you can leave `CORS_ALLOWED_ORIGINS` empty and the session cookie's `SameSite=Lax` does its job.
 - **Set a strong Postgres password** in `.env` before the first `docker compose up`. The defaults are fine for local dev only.
 - **Keep `COOKIE_SECURE=true`** behind HTTPS. Browsers drop `Secure` cookies on plain HTTP; the dev compose accepts `COOKIE_SECURE=false` for that case.
 - **Watch `RUST_LOG`** if you're noisy with `debug` — drop the `,mangalord=debug` suffix in production to log at `info` for the app's spans.