perf: parallelize event + features queries in getOptimizationEventById (#2559)

## Summary
- Runs `optimization_events.findFirst` and
`optimization_features.findUnique` in parallel via `Promise.all`
- The features query only needs `trace_id` (a parameter), not the event
result, making the queries independent
- Wall-clock time goes from sum of both queries to max of either

## Evidence
- Proof doc: `js/cf-webapp/proof/15-parallel-optimization-event.md`

## Test plan
- [ ] `bash
js/cf-webapp/proof/reproducers/15-parallel-optimization-event.sh` — 6/6
checks pass
- [ ] Optimization review page loads correctly with review quality data

js/cf-webapp/proof/15-parallel-optimization-event.md

@@ -0,0 +1,42 @@
+# Proof 15: Parallelize getOptimizationEventById Queries
+
+## What Changed
+In `src/app/(dashboard)/review-optimizations/[traceId]/action.ts`, the `getOptimizationEventById` function previously ran two Prisma queries sequentially:
+1. Fetch the optimization event (with repository include)
+2. Then, if found, fetch `optimization_features` for review quality data
+
+Now both queries run in parallel via `Promise.all`, since the features query only needs `trace_id` (available upfront), not the event result.
+
+## Why
+The two queries are independent — `optimization_features` is looked up by `trace_id`, which is a parameter, not derived from the event result. Running them sequentially wastes wall-clock time equal to the slower query's latency.
+
+**Before**: Total time = event query + features query (sequential)
+**After**: Total time = max(event query, features query) (parallel)
+
+## Evidence
+
+### Before (sequential)
+```typescript
+const event = await prisma.optimization_events.findFirst({ where, include: { repository: true } })
+if (event) {
+  const features = await prisma.optimization_features.findUnique({ where: { trace_id: event.trace_id }, ... })
+  return { ...event, review_quality: features?.review_quality || null, ... }
+}
+```
+
+### After (parallel)
+```typescript
+const [event, features] = await Promise.all([
+  prisma.optimization_events.findFirst({ where, include: { repository: true } }),
+  prisma.optimization_features.findUnique({ where: { trace_id }, select: { review_quality: true, review_explanation: true } }),
+])
+return { ...event, review_quality: features?.review_quality || null, ... }
+```
+
+Key insight: the features query uses `trace_id` (the function parameter), not `event.trace_id` (the result). This makes the queries truly independent.
+
+## How to Verify
+```bash
+cd js/cf-webapp
+bash proof/reproducers/15-parallel-optimization-event.sh
+```

js/cf-webapp/proof/reproducers/15-parallel-optimization-event.sh

@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+# Proof reproducer for commit 15: parallelize getOptimizationEventById
+set -euo pipefail
+
+PASS=0
+FAIL=0
+TOTAL=0
+
+check() {
+  local desc="$1"; shift
+  TOTAL=$((TOTAL + 1))
+  if "$@" >/dev/null 2>&1; then
+    echo "  PASS: $desc"
+    PASS=$((PASS + 1))
+  else
+    echo "  FAIL: $desc"
+    FAIL=$((FAIL + 1))
+  fi
+}
+
+check_not() {
+  local desc="$1"; shift
+  TOTAL=$((TOTAL + 1))
+  if "$@" >/dev/null 2>&1; then
+    echo "  FAIL: $desc"
+    FAIL=$((FAIL + 1))
+  else
+    echo "  PASS: $desc"
+    PASS=$((PASS + 1))
+  fi
+}
+
+ACTION_FILE="src/app/(dashboard)/review-optimizations/[traceId]/action.ts"
+
+echo "=== Proof 15: parallelize getOptimizationEventById ==="
+echo ""
+
+echo "--- Parallelization checks ---"
+check "Promise.all used in getOptimizationEventById" \
+  grep -q 'Promise.all' "$ACTION_FILE"
+
+check "event and features destructured from Promise.all" \
+  grep -q '\[event, features\].*await Promise.all' "$ACTION_FILE"
+
+check "optimization_events.findFirst inside Promise.all block" \
+  grep -q 'optimization_events.findFirst' "$ACTION_FILE"
+
+check "optimization_features.findUnique inside Promise.all block" \
+  grep -q 'optimization_features.findUnique' "$ACTION_FILE"
+
+echo ""
+echo "--- Independence check ---"
+check "features query uses trace_id param directly" \
+  grep -q 'where: { trace_id }' "$ACTION_FILE"
+
+check_not "no sequential if(event) then features pattern" \
+  grep -q 'if (event)' "$ACTION_FILE"
+
+echo ""
+echo "=== Results: $PASS/$TOTAL passed, $FAIL failed ==="
+[ "$FAIL" -eq 0 ] && echo "ALL CHECKS PASSED" || echo "SOME CHECKS FAILED"
+exit "$FAIL"

js/cf-webapp/src/app/(dashboard)/review-optimizations/[traceId]/action.ts

@@ -157,42 +157,44 @@ export async function getOptimizationEventById({
     trace_id,
     ...buildOptimizationOrCondition(payload, repoIds),
   }
-  const event = await prisma.optimization_events.findFirst({
-    where,
-    include: {
-      repository: true,
-    },
-  })
 
-  if (event) {
-    // Fetch review_quality and review_explanation from optimization_features
-    const features = await prisma.optimization_features.findUnique({
-      where: { trace_id: event.trace_id },
+  // Fire both queries in parallel — features only needs trace_id, not the event result
+  const [event, features] = await Promise.all([
+    prisma.optimization_events.findFirst({
+      where,
+      include: {
+        repository: true,
+      },
+    }),
+    prisma.optimization_features.findUnique({
+      where: { trace_id },
       select: {
         review_quality: true,
         review_explanation: true,
       },
-    })
+    }),
+  ])
 
-    // Track that this optimization was reviewed
-    const userId = "userId" in payload ? payload.userId : undefined
-    if (userId) {
-      trackOptimizationReviewed(userId, {
-        traceId: event.trace_id,
-        functionName: event.function_name,
-        repositoryName: event.repository?.full_name ?? null,
-        status: event.status,
-      })
-    }
-
-    return {
-      ...event,
-      review_quality: features?.review_quality || null,
-      review_explanation: features?.review_explanation || null,
-    }
+  if (!event) {
+    return null
   }
 
-  return event
+  // Track that this optimization was reviewed
+  const userId = "userId" in payload ? payload.userId : undefined
+  if (userId) {
+    trackOptimizationReviewed(userId, {
+      traceId: event.trace_id,
+      functionName: event.function_name,
+      repositoryName: event.repository?.full_name ?? null,
+      status: event.status,
+    })
+  }
+
+  return {
+    ...event,
+    review_quality: features?.review_quality || null,
+    review_explanation: features?.review_explanation || null,
+  }
 }
 export async function saveOptimizationChanges({
   eventId,