remotes: support multiple base_urls with round-robin + failover

A remote's base_url may now be a single string OR a list of upstream
mirrors. When it is a list the shared proxy engine load-balances across
them round-robin and, on an upstream error/timeout/5xx, fails over to the
next mirror before returning an error. Because the selection happens in
the engine (not per provider), it applies to every remote package type.
Backward compatible: a bare-string base_url behaves exactly as before.

- add models.StringOrSlice (string-or-array JSON) and custom Remote
  (Un)MarshalJSON: base_url populates BaseURLs (full list) + BaseURL
  (active/first); marshals a single mirror back to a bare string
- add Remote.BaseURLList / ValidateBaseURLs; validate list is non-empty
  and every entry is an http/https URL in the v2 create/update handlers
- persist the full list in a new base_urls TEXT[] column (additive
  migration), keeping base_url in sync for old readers; only write
  base_urls for genuinely multi-mirror remotes
- engine: per-remote round-robin cursor + attempt ordering; wrap the
  fetch/head/revalidate upstream calls in a failover loop that narrows
  the remote to one selected mirror per attempt; only network errors and
  5xx fail over (404/403/... are returned as-is); circuit breaker stays
  keyed per remote and trips only after all mirrors fail
- add unit tests (JSON round-trip, engine round-robin/failover/single-URL,
  DB multi-URL round-trip) and a docker acceptance suite: round-robin
  distribution across two mock upstreams, failover past a dead primary,
  single-base_url regression, and a real dnf makecache+install through a
  two-mirror rpm remote whose primary is dead

Least-connections and a per-remote strategy selector are a follow-up PR.

e2e-docker/README.md

@@ -30,6 +30,12 @@ already-running stack.
   index), rpm (real package + **automatic repodata** generation).
 - **Virtual repositories** — pypi simple-index merge and helm `index.yaml` merge
   across two members.
+- **Multiple base_urls** — a remote whose `base_url` is a list of two upstreams:
+  round-robin distribution across both mirrors (constant-body `mockupstreama` /
+  `mockupstreamb`), failover past a dead primary, single-base_url regression, and
+  a real `dnf` (stock `rockylinux:9` container) `makecache` + `install` through a
+  two-mirror rpm remote with a dead primary — a dead mirror must not break the
+  client.
 
 ## Fixtures