feat(backend): add GET /api/context endpoint (#65)

* feat(backend): add GET /api/context endpoint

Query context values by key with exact/prefix match
support. Default mode tries exact first, falls back
to prefix.

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

* fix(backend): validate context key element types

Reject booleans, nulls, and nested arrays in the key
param. Only string, number, and plain objects with
primitive values are accepted.

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

---------

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

apps/aelis-backend/src/engine/http.ts

@@ -1,5 +1,6 @@
 import type { Context, Hono } from "hono"
 
+import { contextKey } from "@aelis/core"
 import { createMiddleware } from "hono/factory"
 
 import type { AuthSessionMiddleware } from "../auth/session-middleware.ts"
@@ -26,6 +27,7 @@ export function registerFeedHttpHandlers(
 	})
 
 	app.get("/api/feed", inject, authSessionMiddleware, handleGetFeed)
+	app.get("/api/context", inject, authSessionMiddleware, handleGetContext)
 }
 
 async function handleGetFeed(c: Context<Env>) {
@@ -43,3 +45,74 @@ async function handleGetFeed(c: Context<Env>) {
 		})),
 	})
 }
+
+function handleGetContext(c: Context<Env>) {
+	const keyParam = c.req.query("key")
+	if (!keyParam) {
+		return c.json({ error: 'Invalid or missing "key" parameter: must be a JSON array' }, 400)
+	}
+
+	let parsed: unknown
+	try {
+		parsed = JSON.parse(keyParam)
+	} catch {
+		return c.json({ error: 'Invalid or missing "key" parameter: must be a JSON array' }, 400)
+	}
+
+	if (!Array.isArray(parsed) || parsed.length === 0 || !parsed.every(isContextKeyPart)) {
+		return c.json({ error: 'Invalid or missing "key" parameter: must be a JSON array' }, 400)
+	}
+
+	const matchParam = c.req.query("match")
+	if (matchParam !== undefined && matchParam !== "exact" && matchParam !== "prefix") {
+		return c.json({ error: 'Invalid "match" parameter: must be "exact" or "prefix"' }, 400)
+	}
+
+	const user = c.get("user")!
+	const sessionManager = c.get("sessionManager")
+	const session = sessionManager.getOrCreate(user.id)
+	const context = session.engine.currentContext()
+	const key = contextKey(...parsed)
+
+	if (matchParam === "exact") {
+		const value = context.get(key)
+		if (value === undefined) {
+			return c.json({ error: "Context key not found" }, 404)
+		}
+		return c.json({ match: "exact", value })
+	}
+
+	if (matchParam === "prefix") {
+		const entries = context.find(key)
+		if (entries.length === 0) {
+			return c.json({ error: "Context key not found" }, 404)
+		}
+		return c.json({ match: "prefix", entries })
+	}
+
+	// Default: single find() covers both exact and prefix matches
+	const entries = context.find(key)
+	if (entries.length === 0) {
+		return c.json({ error: "Context key not found" }, 404)
+	}
+
+	// If exactly one result with the same key length, treat as exact match
+	if (entries.length === 1 && entries[0]!.key.length === parsed.length) {
+		return c.json({ match: "exact", value: entries[0]!.value })
+	}
+
+	return c.json({ match: "prefix", entries })
+}
+
+/** Validates that a value is a valid ContextKeyPart (string, number, or plain object of primitives). */
+function isContextKeyPart(value: unknown): boolean {
+	if (typeof value === "string" || typeof value === "number") {
+		return true
+	}
+	if (typeof value === "object" && value !== null && !Array.isArray(value)) {
+		return Object.values(value).every(
+			(v) => typeof v === "string" || typeof v === "number" || typeof v === "boolean",
+		)
+	}
+	return false
+}