remotes: add least-connections mirror strategy (round-robin remains default)

Add a per-remote mirror_strategy selector for the mirrorlist, supporting
round_robin (default, unchanged) and least_conn.

- models.Remote gains MirrorStrategy string + MirrorStrategy{RoundRobin,LeastConn}
  constants and ValidateMirrorStrategy (enum check; least_conn requires a
  non-empty mirrorlist). Empty behaves as round_robin for back-compat.
- DB: additive mirror_strategy TEXT NOT NULL DEFAULT 'round_robin' column
  (CREATE TABLE + ADD COLUMN IF NOT EXISTS), wired through remoteCols/scanRemote/
  CreateRemote/UpdateRemote; empty normalized to round_robin on write.
- Engine: least_conn starts each attempt with the pool URL holding the fewest
  in-flight requests via a per-remote/per-URL atomic gauge (incremented around
  each upstream call in head/fetch/checkUpstream), ties broken by the existing
  round-robin rotation. Round-robin path and failover order unchanged;
  single-URL pools are a no-op.
- Tests: unit tests for least-loaded selection, round-robin default, gauge
  inc/dec, single-URL no-op, and strategy validation; DB round-trip covers the
  new column; docker e2e adds a least_conn distribution test and a real dnf
  install through a least_conn remote.

internal/database/database_test.go

@@ -120,6 +120,20 @@ func TestRemoteMirrorlistRoundTrip(t *testing.T) {
 	if len(got.Mirrorlist) != 2 || got.Mirrorlist[0] != mirrors[0] || got.Mirrorlist[1] != mirrors[1] {
 		t.Fatalf("Mirrorlist round-trip = %v, want %v", got.Mirrorlist, mirrors)
 	}
+	// An unset strategy is stored as the round_robin default.
+	if got.MirrorStrategy != models.MirrorStrategyRoundRobin {
+		t.Fatalf("MirrorStrategy default = %q, want %q", got.MirrorStrategy, models.MirrorStrategyRoundRobin)
+	}
+
+	// Updating to least_conn round-trips.
+	got.MirrorStrategy = models.MirrorStrategyLeastConn
+	if err := testDB.UpdateRemote(ctx(), got); err != nil {
+		t.Fatalf("update to least_conn: %v", err)
+	}
+	got, _ = testDB.GetRemote(ctx(), "r-mirror")
+	if got.MirrorStrategy != models.MirrorStrategyLeastConn {
+		t.Fatalf("MirrorStrategy after update = %q, want least_conn", got.MirrorStrategy)
+	}
 
 	// Clearing the mirrorlist on update persists an empty list.
 	got.Mirrorlist = nil