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>

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