api: pair the mobile app with device tokens

MB1 (spec 13). Decision 0005: our own Android/iPhone app, Expo in the
mobile repo, user portal only, no push in v1.

- mobile_devices (migration 0013): one-time pairing codes from My settings
  (QR or server + code, 10 minutes), traded by the app for an spm_ device
  token that acts as its user and never as an admin
- POST /api/me/mobile-devices/pair (web session only), POST /api/mobile/pair,
  GET/DELETE /api/me/mobile-devices
- signing out everywhere (password change or reset, disabling) removes
  paired phones too
- My settings: Mobile app section with the QR code and paired phones
- mobile/ (its own repo) ignored here

docs/decisions/0005-mobile-app.md

@@ -0,0 +1,19 @@
+# 0005 — Our own mobile app: React Native (Expo) in the `mobile` repo
+
+Owner, 2026-10-26: build an Android and iPhone app instead of only recommending Linphone (this
+reverses the "native mobile apps" non-goal in 00 §2). One codebase for both, in its own repo
+(`git.dou.bet/spng/mobile`, AGPL-3.0), with this repo as a git submodule for
+`@selfphoneng/shared`. **Expo (React Native)** because the web client is already React 19 +
+TypeScript + react-query + zustand + zod: the types, schemas and the SIP.js call code carry over,
+which Flutter or Kotlin Multiplatform would not allow. Calls use **SIP.js + react-native-webrtc**
+(both MIT) against the browser softphone's existing `-web` endpoint, so Asterisk stays the only SIP
+stack; Linphone SDK and PJSIP were rejected (GPL with a commercial dual license, which conflicts
+with App Store terms, and their React Native wrappers are abandoned). The native call screen is
+**expo-callkit-telecom** (MIT: CallKit and Jetpack Core-Telecom); react-native-callkeep is
+unmaintained and uses the older ConnectionService. v1 has **no push**: the app rings only while
+open, and CFNA to a mobile number is the fallback (07 §5). Push comes later as FCM (Play) and
+UnifiedPush (F-Droid) on Android and a relay for Apple's VoIP push; that adds a closed-source
+dependency (Play Services) to the Play build, which needs its own decision then. iOS builds without
+a Mac use **ios-builder** (MobAI-App, MIT), which runs Xcode on a macOS CI runner. For now the app
+is built and sideloaded only; App Store and Play listings, and the AGPL/App Store terms question,
+come later.