feat(web): stamp login_events on every gated request

The registration gate, once it resolves the authenticated subject to a tapir
user_id, calls StampLogin (store-throttled to one row per user per day). Best-
effort: a stamp failure is logged and swallowed so it never breaks the request.
This is what makes the read-side Stage-0 usage signal actually accrue.

Tests cover the happy-path stamp, the same-day throttle, and that an
unregistered subject (redirected to /register) is never stamped.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/handlers.go

@@ -34,6 +34,11 @@ type Store interface {
 	DeleteConnection(ctx context.Context, userID, provider string) error
 	DeleteUser(ctx context.Context, userID string) error
 	DisplayName(ctx context.Context, userID string) (string, error)
+
+	// StampLogin records (throttled, one row per user per day) that the resolved
+	// user was active on this request — the read-side Stage-0 usage signal the
+	// registration gate stamps for every authenticated request.
+	StampLogin(ctx context.Context, userID string) error
 }
 
 // SecretRemover deletes secret material by its opaque ref. *secrets.FileStore