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>

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