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

internal/repository/migration.go

@@ -21,13 +21,14 @@ import (
 // is added to internal/repository/migrations/sqlite3/.
 //
 // Version history:
+//   - Version 14: Partial covering indexes for running jobs (tiny B-tree vs millions)
 //   - Version 13: Add covering indexes for status/dashboard queries (cluster, job_state, ...)
 //   - Version 12: Add covering index for stats queries (cluster, start_time, hpc_user, ...)
 //   - Version 11: Optimize job table indexes (reduce from ~78 to 48)
 //   - Version 10: Node table
 //
 // Migration files are embedded at build time from the migrations directory.
-const Version uint = 13
+const Version uint = 14
 
 //go:embed migrations/*
 var migrationFiles embed.FS