feat(api): enforce CPU/memory/storage quotas (spec §9.3, §22)

Add per-user resource quota enforcement across all four dimensions:
max_servers, max_cpu_milli, max_memory_mb, and max_storage_gb.

- Migration 0013: add cached_cpu_milli, cached_memory_mb, cached_storage_mb
  columns to servers table for pure-SQL per-owner aggregation
- SeedServer now writes resource cache alongside server row
- QuotaCheck replaces QuotaAvailable at claim time, checking all four caps
  against the owning user's cumulative usage
- handlePatchServer checks owner's quota before allowing memory/resource
  changes on owned servers; unowned servers skip the gate
- handleInternalClaim mirrors the full quota check
- UpdateServerResources keeps the cache in sync after spec mutations
- Reaper zeros resource cache on ReleaseWorld so released resources
  are not counted against a former owner
- quantityToMilli/quantityToMB helpers convert K8s quantities to
  quota-comparable integers

19 test packages pass.

internal/reaper/pgstore.go

@@ -72,11 +72,12 @@ func (s *PGStore) InsertBackup(ctx context.Context, rec BackupRecord) error {
 	return err
 }
 
-// ReleaseWorld releases ownership and resets the activity clock and warnings —
-// without deleting the row (red line ②).
+// ReleaseWorld releases ownership, zeros the resource cache, and resets the
+// activity clock and warnings — without deleting the row (red line ②).
 func (s *PGStore) ReleaseWorld(ctx context.Context, name string, at time.Time) error {
 	const q = `UPDATE servers
-		SET owner_id = NULL, last_active_at = $2, warned_3d_at = NULL, warned_1d_at = NULL
+		SET owner_id = NULL, cached_cpu_milli = 0, cached_memory_mb = 0, cached_storage_mb = 0,
+		    last_active_at = $2, warned_3d_at = NULL, warned_1d_at = NULL
 		WHERE name = $1 AND deleted_at IS NULL`
 	_, err := s.db.ExecContext(ctx, q, name, at)
 	return err