refactor(server): drop stale prompt text, duplicated routes and dead options

- lib/system-prompts.ts: delete the extended prompt kept only to reach
  the 4096-token cache threshold of two old Claude models; describe the
  current canvas-first UI in the App Features section
- app/api/chat/route.ts: remove the server-side example cache (the
  browser answers examples itself), the ENABLE_HISTORY_XML_REPLACE
  option and its helper, the unread messageMetadata fields; the
  DEBUG_LLM_PAYLOAD blocks become one describeMessages summary
- app/api/log-feedback and log-save share scoreLatestTrace in
  lib/langfuse.ts
- app/api/admin/test-model and validate-model call a shared
  testModelConnection() instead of one route faking a request to the
  other and passing trust through the body
- lib/dynamo-quota-manager.ts reads the failing item from the
  conditional update instead of a second GetItem
- lib/ai-providers.ts: geminiThinkingOptions helper for the google and
  vertexai cases, drop the unused fourth resolveBaseURL parameter
- edge-functions: always stream, shrink messageSchema, drop model
  aliases nothing sends
- delete app/api/chat/xml_guide.md, parseOutputTokenLimit, unused type
  exports and export keywords on file-internal symbols
- env.example: drop variables nothing reads, document the quota,
  validation and analytics variables, mention the apiKeyEnv array form
  (also in docs/*/ai-providers.md)

app/api/log-feedback/route.ts

@@ -1,6 +1,5 @@
-import { randomUUID } from "crypto"
 import { z } from "zod"
-import { getLangfuseClient } from "@/lib/langfuse"
+import { getLangfuseClient, scoreLatestTrace } from "@/lib/langfuse"
 import { getUserIdFromRequest } from "@/lib/user-id"
 
 const feedbackSchema = z.object({
@@ -37,75 +36,24 @@ export async function POST(req: Request) {
     const userId = getUserIdFromRequest(req)
 
     try {
-        // Find the most recent chat trace for this session to attach the score to
-        const tracesResponse = await langfuse.api.trace.list({
+        // Score the session's latest chat trace; without one, a standalone
+        // feedback trace carries the score
+        await scoreLatestTrace(langfuse, {
             sessionId,
-            limit: 1,
+            name: "user-feedback",
+            value: feedback === "good" ? 1 : 0,
+            comment: `User gave ${feedback} feedback`,
+            fallbackTrace: {
+                name: "user-feedback",
+                userId,
+                input: { messageId, feedback },
+                metadata: {
+                    source: "feedback-button",
+                    note: "standalone - no chat trace found",
+                },
+            },
         })
 
-        const traces = tracesResponse.data || []
-        const latestTrace = traces[0]
-
-        if (!latestTrace) {
-            // No trace found for this session - create a standalone feedback trace
-            const traceId = randomUUID()
-            const timestamp = new Date().toISOString()
-
-            await langfuse.api.ingestion.batch({
-                batch: [
-                    {
-                        type: "trace-create",
-                        id: randomUUID(),
-                        timestamp,
-                        body: {
-                            id: traceId,
-                            name: "user-feedback",
-                            sessionId,
-                            userId,
-                            input: { messageId, feedback },
-                            metadata: {
-                                source: "feedback-button",
-                                note: "standalone - no chat trace found",
-                            },
-                            timestamp,
-                        },
-                    },
-                    {
-                        type: "score-create",
-                        id: randomUUID(),
-                        timestamp,
-                        body: {
-                            id: randomUUID(),
-                            traceId,
-                            name: "user-feedback",
-                            value: feedback === "good" ? 1 : 0,
-                            comment: `User gave ${feedback} feedback`,
-                        },
-                    },
-                ],
-            })
-        } else {
-            // Attach score to the existing chat trace
-            const timestamp = new Date().toISOString()
-
-            await langfuse.api.ingestion.batch({
-                batch: [
-                    {
-                        type: "score-create",
-                        id: randomUUID(),
-                        timestamp,
-                        body: {
-                            id: randomUUID(),
-                            traceId: latestTrace.id,
-                            name: "user-feedback",
-                            value: feedback === "good" ? 1 : 0,
-                            comment: `User gave ${feedback} feedback`,
-                        },
-                    },
-                ],
-            })
-        }
-
         return Response.json({ success: true, logged: true })
     } catch (error) {
         console.error("Langfuse feedback error:", error)