feat(backend): add LLM-powered feed enhancement (#58)

* feat(backend): add LLM-powered feed enhancement

Add enhancement harness that fills feed item slots and
generates synthetic items via OpenRouter.

- LLM client with 30s timeout, reusable SDK instance
- Prompt builder with mini calendar and week overview
- arktype schema validation + JSON Schema for structured output
- Pure merge function with clock injection
- Defensive fallback in feed endpoint on enhancement failure
- Skips LLM call when no unfilled slots or no API key

Co-authored-by: Ona <no-reply@ona.com>

* refactor: move feed enhancement into UserSession

Move enhancement logic from HTTP handler into UserSession so the
transport layer has no knowledge of enhancement. UserSession.feed()
handles refresh, enhancement, and caching in one place.

- UserSession subscribes to engine updates and re-enhances eagerly
- Enhancement cache tracks source identity to prevent stale results
- UserSessionManager accepts config object with optional enhancer
- HTTP handler simplified to just call session.feed()

Co-authored-by: Ona <no-reply@ona.com>

* test: add schema sync tests for arktype/JSON Schema drift

Validates reference payloads against both the arktype schema
(parseEnhancementResult) and the OpenRouter JSON Schema structure.
Catches field additions/removals or type changes in either schema.

Co-authored-by: Ona <no-reply@ona.com>

* refactor: rename arktype schemas to match types

Co-authored-by: Ona <no-reply@ona.com>

---------

Co-authored-by: Ona <no-reply@ona.com>

apps/aris-backend/src/session/user-session.ts

@@ -1,24 +1,104 @@
-import { FeedEngine, type FeedSource } from "@aris/core"
+import { FeedEngine, type FeedItem, type FeedResult, type FeedSource } from "@aris/core"
+
+import type { FeedEnhancer } from "../enhancement/enhance-feed.ts"
 
 export class UserSession {
 	readonly engine: FeedEngine
 	private sources = new Map<string, FeedSource>()
+	private readonly enhancer: FeedEnhancer | null
+	private enhancedItems: FeedItem[] | null = null
+	/** The FeedResult that enhancedItems was derived from. */
+	private enhancedSource: FeedResult | null = null
+	private enhancingPromise: Promise<void> | null = null
+	private unsubscribe: (() => void) | null = null
 
-	constructor(sources: FeedSource[]) {
+	constructor(sources: FeedSource[], enhancer?: FeedEnhancer | null) {
 		this.engine = new FeedEngine()
+		this.enhancer = enhancer ?? null
 		for (const source of sources) {
 			this.sources.set(source.id, source)
 			this.engine.register(source)
 		}
+
+		if (this.enhancer) {
+			this.unsubscribe = this.engine.subscribe((result) => {
+				this.invalidateEnhancement()
+				this.runEnhancement(result)
+			})
+		}
+
 		this.engine.start()
 	}
 
+	/**
+	 * Returns the current feed, refreshing if the engine cache expired.
+	 * Enhancement runs eagerly on engine updates; this method awaits
+	 * any in-flight enhancement or triggers one if needed.
+	 */
+	async feed(): Promise<FeedResult> {
+		const cached = this.engine.lastFeed()
+		const result = cached ?? (await this.engine.refresh())
+
+		if (!this.enhancer) {
+			return result
+		}
+
+		// Wait for any in-flight background enhancement to finish
+		if (this.enhancingPromise) {
+			await this.enhancingPromise
+		}
+
+		// Serve cached enhancement only if it matches the current engine result
+		if (this.enhancedItems && this.enhancedSource === result) {
+			return { ...result, items: this.enhancedItems }
+		}
+
+		// Stale or missing — re-enhance
+		await this.runEnhancement(result)
+
+		if (this.enhancedItems) {
+			return { ...result, items: this.enhancedItems }
+		}
+
+		return result
+	}
+
 	getSource<T extends FeedSource>(sourceId: string): T | undefined {
 		return this.sources.get(sourceId) as T | undefined
 	}
 
 	destroy(): void {
+		this.unsubscribe?.()
+		this.unsubscribe = null
 		this.engine.stop()
 		this.sources.clear()
+		this.invalidateEnhancement()
+		this.enhancingPromise = null
+	}
+
+	private invalidateEnhancement(): void {
+		this.enhancedItems = null
+		this.enhancedSource = null
+	}
+
+	private runEnhancement(result: FeedResult): Promise<void> {
+		const promise = this.enhance(result)
+		this.enhancingPromise = promise
+		promise.finally(() => {
+			if (this.enhancingPromise === promise) {
+				this.enhancingPromise = null
+			}
+		})
+		return promise
+	}
+
+	private async enhance(result: FeedResult): Promise<void> {
+		try {
+			this.enhancedItems = await this.enhancer!(result.items)
+			this.enhancedSource = result
+		} catch (err) {
+			console.error("[enhancement] Unexpected error:", err)
+			this.invalidateEnhancement()
+		}
 	}
 }