fix: replace N+1 API calls in MyTreesPage with 2 parallel requests

The page was making 1 API call per tree to fetch session stats, but the
backend sessions endpoint doesn't support tree_id filtering — so every
call returned identical data. Now fetches trees and sessions in parallel
(2 calls total), builds a lastUsed map client-side, and uses the existing
usage_count field from the tree list response for session counts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/MyTreesPage.tsx

@@ -38,33 +38,26 @@ export function MyTreesPage() {
     if (!user?.id) return
     setIsLoading(true)
     try {
-      // Get user's trees (authored by current user)
-      const userTrees = await treesApi.list({ author_id: user.id })
+      // Fetch trees and recent sessions in parallel (2 API calls total, not N+1)
+      const [userTrees, recentSessions] = await Promise.all([
+        treesApi.list({ author_id: user.id }),
+        sessionsApi.list({ size: 100 }),
+      ])
 
-      // Load session stats for each tree
-      const treesWithStats = await Promise.all(
-        userTrees.map(async (tree) => {
-          try {
-            const sessions = await sessionsApi.list({ tree_id: tree.id })
-            const lastUsed = sessions.length > 0
-              ? sessions.reduce((latest, session) =>
-                  new Date(session.started_at) > new Date(latest.started_at) ? session : latest
-                ).started_at
-              : undefined
-            return {
-              ...tree,
-              lastUsed,
-              sessionCount: sessions.length,
-            }
-          } catch (err) {
-            console.error(`Failed to load stats for tree ${tree.id}:`, err)
-            return {
-              ...tree,
-              sessionCount: 0,
-            }
-          }
-        })
-      )
+      // Build a map of tree_id -> most recent session start time
+      const lastUsedMap = new Map<string, string>()
+      for (const session of recentSessions) {
+        const existing = lastUsedMap.get(session.tree_id)
+        if (!existing || new Date(session.started_at) > new Date(existing)) {
+          lastUsedMap.set(session.tree_id, session.started_at)
+        }
+      }
+
+      const treesWithStats: TreeWithStats[] = userTrees.map((tree) => ({
+        ...tree,
+        lastUsed: lastUsedMap.get(tree.id),
+        sessionCount: tree.usage_count ?? 0,
+      }))
 
       setTrees(treesWithStats)
     } catch (err) {