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/Models/Models.swift

@@ -50,6 +50,9 @@ struct StorySummary: Codable, Identifiable, Hashable {
     let headline: String
     let summary: String
     let topic: String
+    /// Canonical category slugs from the backend. Optional so decoding tolerates
+    /// the transition while the backend rolls out `tags[]`; falls back to `topic`.
+    let tags: [String]?
     let signalScore: Int
     let scoreBreakdown: ScoreBreakdown
     let sourceCount: Int