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

cmd/cc-backend/server.go

@@ -20,6 +20,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/99designs/gqlgen/graphql"
 	"github.com/99designs/gqlgen/graphql/handler"
 	"github.com/99designs/gqlgen/graphql/handler/transport"
 	"github.com/99designs/gqlgen/graphql/playground"
@@ -89,6 +90,12 @@ func (s *Server) init() error {
 
 	graphQLServer.AddTransport(transport.POST{})
 
+	// Inject a per-request stats cache so that grouped statistics queries
+	// sharing the same (filter, groupBy) pair are executed only once.
+	graphQLServer.AroundOperations(func(ctx context.Context, next graphql.OperationHandler) graphql.ResponseHandler {
+		return next(graph.WithStatsGroupCache(ctx))
+	})
+
 	if os.Getenv(envDebug) != "1" {
 		// Having this handler means that a error message is returned via GraphQL instead of the connection simply beeing closed.
 		// The problem with this is that then, no more stacktrace is printed to stderr.