Release 0.10.0: user-set passcodes (OTC stays as fallback)

After a successful OTC sign-in, a contributor can set a passcode
(4-20 characters, bcrypt-hashed) and use email + passcode for
subsequent sign-ins. OTC remains the structural fallback: five
consecutive failed verifies lock the passcode path for 15 minutes
(HTTP 423), and a forgotten passcode is recovered by requesting a
fresh code. Migration 015_passcode.sql adds four nullable columns
to the users table; existing rows pass through as OTC-only and can
opt into a passcode from a new Sign-in tab in /settings/notifications.
The /login surface is extended to a five-step flow (email → either
passcode or OTC code → optional post-OTC passcode offer → optional
set-passcode). SPEC corrections per §19.3 rule 2: §6 names the
three auth paths, §14.1 documents the stepped login flow, §17 lists
the four new /auth/passcode/* endpoints, §19.2 surfaces four new
candidates and refreshes the cross-refs.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.8.0",
+  "version": "0.10.0",
   "type": "module",
   "scripts": {
     "dev": "vite",