Implement email magic-link authentication

internal/auth/ provides:
- TokenStore: 32-byte cryptographically random one-time tokens.
  Only the SHA-256 hash is persisted (so a DB leak doesn't grant
  active sessions). Comparison uses subtle.ConstantTimeCompare.
  Single-use is enforced via UPDATE ... WHERE used_at IS NULL.
- Signer: HS256 JWTs with 24h lifetime, jwt.WithValidMethods to
  reject alg=none and other downgrade attacks.
- LogMailer (dev) and SMTPMailer (prod via net/smtp) behind a
  Mailer interface.
- RateLimiter: DB-backed fixed window per email; default 5 per
  15 min for the magic-link flow.
- Service: orchestrates RequestLogin (auto-creates user on first
  login, generates token, emails magic link) and Verify (consumes
  token, updates last_login, issues JWT).
- Handlers: POST /auth/login and GET/POST /auth/verify.
  HandleLogin returns 202 even on validation failure to avoid
  account enumeration; rate-limit hits surface as 429.

Schema additions: magic_tokens (with FK + cascade) and
login_attempts. UserStore.SetStoragePath added for completeness.

Tests cover: token issue/consume, single-use, expiry, rate limit,
JWT round-trip, alg=none rejection, signature tampering, purge,
HTTP handlers (login + verify, missing/invalid token paths).

Closes #9.

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

internal/auth/mailer.go

@@ -0,0 +1,62 @@
+package auth
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/smtp"
+	"strings"
+)
+
+// Mailer sends magic-link emails. The interface keeps the auth flow
+// decoupled from any concrete delivery backend.
+type Mailer interface {
+	SendMagicLink(ctx context.Context, email, link string) error
+}
+
+// LogMailer writes the magic link to a writer instead of sending an
+// email. Useful for development and tests.
+type LogMailer struct{ W io.Writer }
+
+func (m LogMailer) SendMagicLink(_ context.Context, email, link string) error {
+	if m.W == nil {
+		return nil
+	}
+	_, err := fmt.Fprintf(m.W, "magic link for %s: %s\n", email, link)
+	return err
+}
+
+// SMTPMailer delivers via plain SMTP with optional auth. TLS is the
+// caller's responsibility (use submission port 587 with STARTTLS or 465
+// with implicit TLS — the stdlib smtp package handles STARTTLS via
+// SendMail when the server supports it).
+type SMTPMailer struct {
+	Host     string
+	Port     string
+	Username string
+	Password string
+	From     string
+}
+
+func (m SMTPMailer) SendMagicLink(_ context.Context, to, link string) error {
+	if m.Host == "" || m.From == "" {
+		return fmt.Errorf("smtp not configured")
+	}
+	addr := m.Host + ":" + m.Port
+	var auth smtp.Auth
+	if m.Username != "" {
+		auth = smtp.PlainAuth("", m.Username, m.Password, m.Host)
+	}
+	subject := "Your librenotes login link"
+	body := fmt.Sprintf("Click to sign in to librenotes:\n\n%s\n\nThis link is valid for 15 minutes and can be used once.\n", link)
+	msg := strings.Join([]string{
+		"From: " + m.From,
+		"To: " + to,
+		"Subject: " + subject,
+		"MIME-Version: 1.0",
+		"Content-Type: text/plain; charset=utf-8",
+		"",
+		body,
+	}, "\r\n")
+	return smtp.SendMail(addr, auth, m.From, []string{to}, []byte(msg))
+}