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/storage/db.go

@@ -47,4 +47,18 @@ var migrations = []string{
 		storage_path  TEXT NOT NULL
 	)`,
 	`CREATE INDEX IF NOT EXISTS idx_users_email ON users(email)`,
+	`CREATE TABLE IF NOT EXISTS magic_tokens (
+		token_hash TEXT PRIMARY KEY,
+		user_id    TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+		email      TEXT NOT NULL,
+		created_at INTEGER NOT NULL,
+		expires_at INTEGER NOT NULL,
+		used_at    INTEGER
+	)`,
+	`CREATE INDEX IF NOT EXISTS idx_magic_tokens_expires ON magic_tokens(expires_at)`,
+	`CREATE TABLE IF NOT EXISTS login_attempts (
+		email      TEXT NOT NULL,
+		created_at INTEGER NOT NULL
+	)`,
+	`CREATE INDEX IF NOT EXISTS idx_login_attempts ON login_attempts(email, created_at)`,
 }