Add stage 7: email verification magic links and a profile page

Backend
- New Mail namespace: a Mailer interface with SMTP (phpmailer), PHP mail()
  (the default fallback), and log-to-file transports, selected by
  MAIL_TRANSPORT. EmailVerifier issues a hashed, 15-minute magic-link token
  and sends the link (APP_URL/verify-email?token=...).
- Migration 005: email_verifications table + users.verification_email_sent_at.
- Registration now emails a verification link (best effort — a send failure
  doesn't fail registration).
- POST /api/auth/verify-email consumes a token and returns a session, so
  opening the link verifies the address (or applies a pending email change)
  and logs the user in. Single-use; distinct 400s for invalid/used/expired.
- POST /api/email/verification resends; POST /api/email/change requests a
  deferred change (current password required; link goes to the new address;
  users.email only updates when that link is opened). Both throttled to once
  per 60s, returning 429 + retry_after.
- GET /api/me and every session payload now include pending_email. Shared
  SessionPayload builds the user/session JSON for all entry points.

Frontend
- /verify-email view: posts the token, adopts the returned session, redirects.
- /profile view: shows address + status, a resend button with a live cooldown
  (driven by retry_after / 429), and a change-email form (new address +
  current password) that surfaces the pending change.
- Header shows a "verify email" badge linking to the profile.

Tests: 9 new (EmailVerificationTest) covering the link lifecycle, throttle,
and deferred change; AuthTest folded into ApiTestCase, which now routes mail
to a per-test log. Suite: 32 passing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

migrations/005_add_email_verification_tokens.sql

@@ -0,0 +1,18 @@
+-- Magic-link tokens for verifying an email address. `new_email` is null for a
+-- plain "verify your current address" link, or the requested address for a
+-- deferred email change (applied only when the link is opened).
+CREATE TABLE IF NOT EXISTS email_verifications (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id     INTEGER NOT NULL REFERENCES users (id) ON DELETE CASCADE,
+    token_hash  TEXT NOT NULL UNIQUE,
+    new_email   TEXT NULL,
+    expires_at  TEXT NOT NULL,
+    consumed_at TEXT NULL,
+    created_at  TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_email_verifications_user ON email_verifications (user_id);
+
+-- When the last verification / change email was sent to this user, for the
+-- once-per-minute resend throttle.
+ALTER TABLE users ADD COLUMN verification_email_sent_at TEXT NULL;