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>

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/gdamore/encoding v1.0.1 // indirect
 	github.com/gdamore/tcell/v2 v2.8.1 // indirect
+	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/junegunn/go-shellwords v0.0.0-20240813092932-a62c48c52e97 // indirect
 	github.com/lucasb-eyer/go-colorful v1.2.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect