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.

e2e-docker/README.md

@@ -36,6 +36,12 @@ already-running stack.
   no-mirrorlist regression, and a real `dnf` (stock `rockylinux:9` container)
   `makecache` + `install` through a two-mirror rpm remote whose `base_url` is dead
   — a dead mirror must not break the client.
+- **Mirror strategy (`least_conn`)** — a `mirror_strategy: least_conn` rpm remote
+  over the two constant-body mirrors exercises the least-connections selection
+  path end-to-end (both mirrors serve, all requests succeed), plus a real `dnf`
+  install through a `least_conn` remote with a dead primary (failover unchanged).
+  The precise least-loaded pick is asserted deterministically in the proxy unit
+  test, since an in-flight-skew assertion over HTTP is timing-sensitive.
 
 ## Fixtures