feat(stories): StoryCard + spoiler shield

[ArchILLtect]

Summary

• If triggers.length > 0, hide content behind a shield with badges and a Reveal button.

Motivation / Problem

• Safety: flagged content must be user‑revealed; this also provides the core visual element of the feed.

Acceptance Criteria

• Content is hidden when triggers.length > 0; clicking Reveal toggles in place.
• A11y: button has aria-expanded and aria-controls; keyboard activation works; respects reduced‑motion.

Target Release

v0.3.1

Notes / Links

• Files: src/components/StoryCard.tsx, src/components/TriggerBadges.tsx
• Tests: src/__tests__/StoryCardShield.test.tsx