Optimize usage dashboard: partial indexes, request cache, parallel histograms

- Add migration 14: partial covering indexes WHERE job_state='running'
  for user/project/subcluster groupings (tiny B-tree vs full table)
- Inline literal state value in BuildWhereClause so SQLite matches
  partial indexes instead of parameterized placeholders
- Add per-request statsGroupCache (sync.Once per filter+groupBy key)
  so identical grouped stats queries execute only once per GQL operation
- Parallelize 4 histogram queries in AddHistograms using errgroup
- Consolidate frontend from 6 GQL aliases to 2, sort+slice top-10
  client-side via $derived

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Entire-Checkpoint: 5b26a6e5ff10

go.mod

@@ -38,6 +38,7 @@ require (
 	github.com/vektah/gqlparser/v2 v2.5.32
 	golang.org/x/crypto v0.48.0
 	golang.org/x/oauth2 v0.35.0
+	golang.org/x/sync v0.19.0
 	golang.org/x/time v0.14.0
 )
 
@@ -113,7 +114,6 @@ require (
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/mod v0.33.0 // indirect
 	golang.org/x/net v0.51.0 // indirect
-	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/sys v0.41.0 // indirect
 	golang.org/x/text v0.34.0 // indirect
 	golang.org/x/tools v0.42.0 // indirect