docs: M7 push backend contract + status (website#78)

- BACKEND_DESIGN.md: push_devices + notification_subscriptions tables (§3), the
  /auth/me/devices* + /auth/me/notifications/* API rows (§4), a push-notification
  design + security section (content-free tickles, PUBLIC_KINDS split, owner-keyed
  personal streams, SSRF endpoint guard, untrusted-relay model), and the ntfy
  compose service in the deploy section (§8).
- PLAN.md: flip M7 Part 1 (backend + docs) to in-review — status line, §8 item 3,
  §9 M7.

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

website/BACKEND_DESIGN.md

@@ -156,6 +156,28 @@ Seeded keys: `site_mode` (default `maintenance`), `site_mode_changed_at`,
 Same "store only the hash of an opaque token" pattern as `user_invites` / `mobile_refresh_tokens`.
 A DB read never yields a usable reset link. See §4 `/auth/password/*`.
 
+### push_devices  — opt-in push endpoints (M7)
+| col | type | notes |
+|---|---|---|
+| id | INT PK AUTO_INCREMENT | |
+| user_id | INT NOT NULL FK→users(id) ON DELETE CASCADE | owner |
+| transport | ENUM('unifiedpush','fcm') DEFAULT 'unifiedpush' | UnifiedPush for the sideloaded APK; FCM reserved for a later Play flavor |
+| endpoint | VARCHAR(512) NOT NULL | the distributor URL the app's ntfy topic was handed (or an FCM token). Unguessable but **not a secret** — stored in the clear (unlike refresh tokens), because pushes are content-free tickles |
+| platform | VARCHAR(40) NULL | free-form label, e.g. `android` |
+| created_at / last_seen_at | DATETIME | |
+
+`UNIQUE(user_id, endpoint)` — re-registering the same endpoint is an idempotent upsert.
+
+### notification_subscriptions  — which streams a user opted into (M7)
+| col | type | notes |
+|---|---|---|
+| user_id | INT NOT NULL FK→users(id) ON DELETE CASCADE | |
+| stream_id | VARCHAR(64) NOT NULL | an id from the catalog (`config/notificationStreams.js`), validated on write |
+| created_at | DATETIME | |
+
+`PRIMARY KEY(user_id, stream_id)`. Subscriptions are per-user (applied to every device); a PUT
+replaces the whole set. Nothing is pushed unless the user subscribed.
+
 ---
 
 ## 4. API contract
@@ -177,6 +199,10 @@ accepts `Authorization: Bearer` for API testing).
 | PATCH | `/me/account/password` | cookie / bearer (rate-limited) | `{newPassword, currentPassword?}` | change/set own password (current required unless the account has none); revokes other sessions, keeps the caller's |
 | POST | `/me/account/totp/setup` · `…/enable` · `…/disable` | cookie / bearer | `{code}` on enable/disable | self 2FA enrollment (disable needs a valid current code, not a password) |
 | GET | `/me/account/identities` · DELETE `…/:provider` | cookie / bearer | — | list / unlink own SSO identities |
+| POST | `/me/devices` | cookie / bearer | `{endpoint, transport?, platform?}` | register a push endpoint; **rejects a disallowed endpoint 400** (SSRF guard). Idempotent per (user, endpoint) |
+| GET | `/me/devices` · DELETE `…/:id` | cookie / bearer | — | list / unregister own push devices |
+| GET | `/me/notifications/streams` | cookie / bearer | — | the subscribable catalog (`personal`/`requiresLinkedAccount` flags) |
+| GET · PUT | `/me/notifications/subscriptions` | cookie / bearer | `{streams:[id]}` on PUT | get / replace own opted-in streams (unknown ids dropped) |
 
 **Role-agnostic self-service (`/auth/me/*`).** The canonical "me" surface for **every** authenticated
 role. It reuses the exact `account.controller` handlers as `/player/account/*` and `/admin/account/*`
@@ -192,6 +218,26 @@ reset link points at the web front end (`/account/reset/:token`); the Android ap
 rather than shipping its own reset screen (docs/android/PLAN.md §4.2). First admin is bootstrapped
 by `seed.js` from env (see §6); further staff are created under `/admin/users` or via email invites.
 
+**Push notifications (M7, opt-in).** The app subscribes per stream (`/auth/me/notifications/*`) and
+registers device endpoints (`/auth/me/devices`); nothing is pushed unless subscribed. Delivery is a
+**content-free tickle** — `{ stream, ref }`, no sensitive data — POSTed to each subscribed device's
+self-hosted **ntfy** endpoint (`utils/pushDispatch`); the app wakes and pulls the real, ownership-
+checked content over the authenticated API. Two producers fan out through the one publisher: the shard
+ingest dispatcher (`utils/shardIngest`, beside the SSE broadcast) for shard-derived streams, and the
+create/publish-post path for `news.post`. The stream catalog + event→stream mapping is
+`config/notificationStreams.js`. Security invariants:
+- **Same public/admin split as the SSE feed.** Public streams are drawn *only* from the SSE
+  `PUBLIC_KINDS` allowlist; a sensitive kind (audit/cheat/IP/login-attempt) can never produce a public
+  push.
+- **Personal streams are owner-keyed.** `vendor.sale` / `house.idoc` / `account.login` are delivered
+  only to the *owning* user's devices, resolved via `shardLinks` (the same ownership check as
+  `/player/shard/*`).
+- **SSRF guard.** A device `endpoint` is a client-supplied URL the server POSTs to, so registration and
+  every publish validate it is HTTPS, non-private/loopback, and (when configured) on the shard's ntfy
+  allow-set (`NTFY_BASE_URL` / `NTFY_ALLOWED_ORIGINS`).
+- ntfy is treated as an **untrusted relay** — no per-user accounts, unguessable topics; an optional
+  `NTFY_PUBLISH_TOKEN` hardens backend→ntfy publishes but is not required. See docs/android/PLAN.md §11.
+
 ### /public  (public.routes.js → public.controller.js) — all GET, no auth
 | Method | Path | Notes |
 |---|---|---|
@@ -304,7 +350,13 @@ subsystem (`[server]`, `[http]`, `[db]`, `[auth]`, `[admin]`, `[ratelimit]`, …
 - `app`: builds the Dockerfile (installs client+server, builds Vite, serves via Express),
   `env_file: .env`, `DB_HOST=db`, `depends_on: db (healthy)`, volume `uploads:/app/uploads`,
   `ports: "3000:3000"` — **binds 0.0.0.0** (no `127.0.0.1:` prefix) so Pangolin reaches it.
-- Volumes: `dbdata`, `uploads`.
+- `ntfy` (M7): pinned upstream `binwiederhier/ntfy` image, declarative config only
+  (`./ntfy/server.yml` mounted `:ro` + `NTFY_BASE_URL`), volume `ntfydata:/var/lib/ntfy`, **no
+  published host port** — devices reach it via the reverse proxy; the backend publisher reaches it
+  over the private compose network. Anonymous read-write to unguessable topics (no accounts to
+  provision) — safe because pushes are content-free tickles. Bringing the stack up provisions a
+  working push relay with **zero interactive setup**.
+- Volumes: `dbdata`, `uploads`, `ntfydata`.
 
 Express listens on `0.0.0.0:${PORT||3000}`. Pangolin terminates TLS and proxies to `app`.
 
@@ -326,6 +378,9 @@ ADMIN_USERNAME=
 ADMIN_PASSWORD=
 # Email: configured in Admin → Settings → Email (Gmail OAuth2), not via env
 CLIENT_ORIGIN=http://localhost:5173
+# Push (M7): the ntfy relay URL — also the backend's SSRF allow-set for device
+# endpoints. NTFY_ALLOWED_ORIGINS / NTFY_PUBLISH_TOKEN are optional.
+NTFY_BASE_URL=https://ntfy.example.com
 ```
 
 `.gitignore`: `node_modules/`, `.env`, `_reference/`, `client/dist/`, `uploads/`.