Filter pills by backend tags (slug membership), drop keyword matching

Replace the keyword/source-majority Interest system with a StoryPill enum that
matches stories by canonical category slugs. StorySummary gains a tolerant
tags[] (optional, decode-safe); effectiveTags uses tags, falling back to topic
during the backend transition. No more headline keyword-searching on the client —
the backend decides classification; the client asks slug membership.

Pills: All, F1, Sport, AI, Cloud, HomeLab, Tech, Uganda, South Africa, Canada, US.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Jarvis/Store/StoryStore.swift

@@ -129,6 +129,7 @@ final class StoryStore: ObservableObject {
             headline: old.headline,
             summary: old.summary,
             topic: old.topic,
+            tags: old.tags,
             signalScore: signalScore,
             scoreBreakdown: old.scoreBreakdown,
             sourceCount: sourceCount,