remotes: add least-connections mirror strategy (round-robin remains default) (#122)

## Why

The mirrorlist (PR #121) always load-balances round-robin. Round-robin is oblivious to how busy each mirror is, so a slow or saturated mirror keeps getting its fair share of new requests. This adds an opt-in **least-connections** strategy that favors the mirror currently handling the fewest in-flight requests, steering new work toward idle mirrors. Round-robin stays the default, so existing remotes are unchanged.

## How

- **Model**: `models.Remote` gains `mirror_strategy` (`round_robin` default/empty, or `least_conn`). `ValidateMirrorStrategy` checks the enum and requires a non-empty mirrorlist for `least_conn`. 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**: a per-remote, per-pool-URL atomic in-flight gauge is incremented around each upstream call (head/fetch/checkUpstream). For a `least_conn` remote the attempt order starts with the least-loaded pool URL (ties broken by the existing round-robin rotation). Round-robin path and failover order are 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 validation; DB round-trip covers the new column; docker e2e adds a `least_conn` distribution test plus a real `dnf` install through a `least_conn` remote.

Back-compat: unset/empty `mirror_strategy` is `round_robin`, so all existing remotes keep their current behavior.
Reviewed-on: #122
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

pkg/models/remote_test.go

@@ -74,6 +74,30 @@ func TestUpstreamPool(t *testing.T) {
 	}
 }
 
+func TestValidateMirrorStrategy(t *testing.T) {
+	ml := []string{"https://m.example"}
+	cases := []struct {
+		name    string
+		remote  Remote
+		wantErr bool
+	}{
+		{"empty defaults ok", Remote{Mirrorlist: ml}, false},
+		{"explicit round_robin ok", Remote{MirrorStrategy: MirrorStrategyRoundRobin, Mirrorlist: ml}, false},
+		{"round_robin without mirrorlist ok", Remote{MirrorStrategy: MirrorStrategyRoundRobin}, false},
+		{"least_conn with mirrorlist ok", Remote{MirrorStrategy: MirrorStrategyLeastConn, Mirrorlist: ml}, false},
+		{"least_conn without mirrorlist rejected", Remote{MirrorStrategy: MirrorStrategyLeastConn}, true},
+		{"unknown strategy rejected", Remote{MirrorStrategy: "random", Mirrorlist: ml}, true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := tc.remote.ValidateMirrorStrategy()
+			if (err != nil) != tc.wantErr {
+				t.Errorf("ValidateMirrorStrategy() err = %v, wantErr = %v", err, tc.wantErr)
+			}
+		})
+	}
+}
+
 func TestValidateMirrorlist(t *testing.T) {
 	cases := []struct {
 		name    string