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.

internal/api/v2/remotes.go

@@ -74,8 +74,8 @@ func (h *RemotesHandler) create(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, fmt.Sprintf("invalid repo type: %q", remote.RepoType), http.StatusBadRequest)
 		return
 	}
-	if remote.RepoType == models.RepoTypeRemote && remote.BaseURL == "" {
-		http.Error(w, "base_url is required for remote repositories", http.StatusBadRequest)
+	if err := remote.ValidateBaseURLs(); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}
 	if err := remote.ValidatePatterns(); err != nil {
@@ -102,6 +102,10 @@ func (h *RemotesHandler) update(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	remote.Name = name
+	if err := remote.ValidateBaseURLs(); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
 	if err := remote.ValidatePatterns(); err != nil {
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return