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/postgres.go

@@ -45,6 +45,7 @@ func (db *DB) migrate() error {
 			repo_type       TEXT DEFAULT 'remote',
 			base_url        TEXT NOT NULL DEFAULT '',
 			mirrorlist      TEXT[] DEFAULT '{}',
+			mirror_strategy TEXT NOT NULL DEFAULT 'round_robin',
 			description     TEXT DEFAULT '',
 			username        TEXT DEFAULT '',
 			password        TEXT DEFAULT '',
@@ -126,6 +127,7 @@ func (db *DB) migrate() error {
 
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS repo_type TEXT DEFAULT 'remote';
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS mirrorlist TEXT[] DEFAULT '{}';
+		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS mirror_strategy TEXT NOT NULL DEFAULT 'round_robin';
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_dial_timeout INTEGER DEFAULT 0;
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_tls_timeout INTEGER DEFAULT 0;
 		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_response_header_timeout INTEGER DEFAULT 0;