show processing reasons as pills on pipeline cards

Analyzer now computes structured reason tags (Remove tracks, Reorder,
Extract subs, Transcode, Fix default, Fix language tag, Fix title) and
stores them as JSON in review_plans.reasons. Pipeline cards show these
as badges next to the copy/transcode pill so users know why a file
needs processing. Replaces the old transcode_reasons computed from
stream_decisions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/types.ts

@@ -108,6 +108,8 @@ export interface PlanResult {
 		transcode_codec: string | null;
 	}>;
 	notes: string[];
+	/** Short reason tags explaining why the file needs processing. */
+	reasons: string[];
 }
 
 // ─── Scan state ───────────────────────────────────────────────────────────────