feat(auth): mobile-only register/login with OTP verification

- Register and login by mobile number; email is now an optional
  contact field only (never used to authenticate)
- After registration, the phone is verified via a 6-digit SMS code
- Login supports both password and one-time-code (OTP) methods
- Phone OTP delivered via Kavenegar (verify/lookup); API key in env
- Account page: edit name/optional email, change password, and
  change mobile number with OTP re-verification
- Codes are hashed, expire in 5m, capped at 5 attempts, rate-limited
- Seed gives the admin a verified phone so mobile login still works

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/migrations/018_user_phone_and_verification.sql

@@ -0,0 +1,33 @@
+-- Mobile-first auth: phone is the login identifier, email becomes an optional
+-- contact field, plus a table of short-lived one-time SMS codes for verifying
+-- a phone (registration/login completion and number changes).
+
+-- Email becomes optional (login no longer uses it). Postgres treats NULLs as
+-- distinct, so the existing UNIQUE constraint keeps working for users without one.
+ALTER TABLE users ALTER COLUMN email DROP NOT NULL;
+
+ALTER TABLE users ADD COLUMN IF NOT EXISTS phone VARCHAR;
+ALTER TABLE users ADD COLUMN IF NOT EXISTS "phoneVerified" BOOLEAN NOT NULL DEFAULT FALSE;
+
+-- Unique per non-null phone (NULLs allowed for legacy email-only staff accounts).
+CREATE UNIQUE INDEX IF NOT EXISTS users_phone_unique ON users (phone) WHERE phone IS NOT NULL;
+
+-- One-time SMS verification codes (hashed).
+DO $$ BEGIN
+  CREATE TYPE verification_codes_purpose_enum AS ENUM ('login', 'change_phone');
+EXCEPTION WHEN duplicate_object THEN null; END $$;
+
+CREATE TABLE IF NOT EXISTS verification_codes (
+  id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  "userId"    UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  purpose     verification_codes_purpose_enum NOT NULL,
+  destination VARCHAR NOT NULL,
+  "codeHash"  VARCHAR NOT NULL,
+  "expiresAt" TIMESTAMPTZ NOT NULL,
+  attempts    INT NOT NULL DEFAULT 0,
+  "consumedAt" TIMESTAMPTZ,
+  "createdAt" TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS verification_codes_user_purpose_idx
+  ON verification_codes ("userId", purpose);