remotes: add mirrorlist for round-robin + failover across mirrors (rpm/deb/apk) (#121)

## Why
OS package remotes (rpm/deb/apk) fetch many small files and benefit from spreading upstream load across mirrors and surviving a mirror outage. A remote may now set a **`mirrorlist`** of additional upstream base URLs. The effective upstream pool is **`[base_url] + mirrorlist`**, which the shared proxy engine load-balances **round-robin** and, on a network error/timeout/5xx, **fails over** to the next mirror before returning an error. Selection happens in the engine, so it works for every provider that reaches upstream.

**Backward compatible:** `base_url` stays a plain string (providers read it unchanged), and a remote with **no mirrorlist behaves exactly as today** (single attempt, same error path).

## How
- `models.Remote.Mirrorlist` (`[]string`, `json:"mirrorlist,omitempty"`) + `UpstreamPool()` = `[base_url] + mirrorlist`.
- `ValidateMirrorlist`: a non-empty mirrorlist is allowed **only** when `repo_type==remote` **and** `package_type ∈ {rpm, deb, alpine}`; each entry must be an http/https URL. Enforced in the v2 create/update handlers (400 otherwise); `base_url` stays required for remotes.
- Persist the mirrorlist in a new additive `mirrorlist TEXT[]` column (`remoteCols`/`scanRemote`/`CreateRemote`/`UpdateRemote`); the `base_url` column is unchanged.
- Engine keeps a per-remote round-robin cursor over the pool; the fetch/head/revalidate upstream calls run in a failover loop that narrows the remote to one selected mirror per attempt. Only network errors and 5xx fail over (404/403/… return as-is). The circuit breaker stays keyed per remote and trips only after all mirrors fail.

## Scope
Round-robin + failover only, restricted to **remote rpm/deb/apk** repos. Least-connections and a per-remote strategy selector are a **follow-up PR**.

## Tests
- Unit: model JSON round-trip + validation gating (rejected on non-rpm/deb/apk and on local, accepted on rpm/deb/apk, bad URL rejected), engine round-robin/failover/no-mirrorlist-unchanged, DB mirrorlist round-trip. `make test` (`go test -race`) green.
- Docker acceptance (`e2e-docker`, `dockere2e` tag, wired into `docker-e2e.sh`): round-robin distribution across two mock upstreams, failover past a dead primary, no-mirrorlist regression, and a **real `dnf` makecache + install** through a two-mirror rpm remote whose `base_url` is dead. All four pass locally.

Reviewed-on: #121
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

pkg/models/remote.go

@@ -2,6 +2,7 @@ package models
 
 import (
 	"fmt"
+	"net/url"
 	"regexp"
 	"time"
 )
@@ -39,9 +40,13 @@ type Remote struct {
 	PackageType PackageType `json:"package_type"`
 	RepoType    RepoType    `json:"repo_type"`
 	BaseURL     string      `json:"base_url"`
-	Description string      `json:"description,omitempty"`
-	Username    string      `json:"-"`
-	Password    string      `json:"-"`
+	// Mirrorlist holds additional upstream mirror base URLs. The effective
+	// upstream pool is [base_url] + mirrorlist, load-balanced round-robin with
+	// failover by the proxy engine. Only valid on remote rpm/deb/apk repos.
+	Mirrorlist  []string `json:"mirrorlist,omitempty"`
+	Description string   `json:"description,omitempty"`
+	Username    string   `json:"-"`
+	Password    string   `json:"-"`
 
 	ImmutableTTL int  `json:"immutable_ttl"`
 	MutableTTL   int  `json:"mutable_ttl"`
@@ -72,6 +77,51 @@ type Remote struct {
 	UpdatedAt time.Time `json:"updated_at"`
 }
 
+// mirrorlistPackageTypes are the package types for which a mirrorlist is
+// allowed: OS package repos (rpm, deb, apk/alpine) that fetch many small files
+// and benefit most from mirror load-balancing and failover.
+var mirrorlistPackageTypes = map[PackageType]bool{
+	PackageRPM:    true,
+	PackageDeb:    true,
+	PackageAlpine: true,
+}
+
+// UpstreamPool returns the ordered upstream base URLs for this remote: the
+// primary base_url first, followed by any mirrorlist entries. The proxy engine
+// load-balances round-robin across the pool and fails over between them.
+func (r Remote) UpstreamPool() []string {
+	pool := make([]string, 0, 1+len(r.Mirrorlist))
+	if r.BaseURL != "" {
+		pool = append(pool, r.BaseURL)
+	}
+	pool = append(pool, r.Mirrorlist...)
+	return pool
+}
+
+// ValidateMirrorlist enforces that a mirrorlist is only configured on remote
+// rpm/deb/apk repositories and that every entry is a parseable http/https URL.
+func (r *Remote) ValidateMirrorlist() error {
+	if len(r.Mirrorlist) == 0 {
+		return nil
+	}
+	if r.RepoType != RepoTypeRemote {
+		return fmt.Errorf("mirrorlist is only allowed on remote repositories")
+	}
+	if !mirrorlistPackageTypes[r.PackageType] {
+		return fmt.Errorf("mirrorlist is only allowed for rpm, deb and alpine package types, not %q", r.PackageType)
+	}
+	for _, u := range r.Mirrorlist {
+		parsed, err := url.ParseRequestURI(u)
+		if err != nil {
+			return fmt.Errorf("invalid mirrorlist url %q: %w", u, err)
+		}
+		if parsed.Scheme != "http" && parsed.Scheme != "https" {
+			return fmt.Errorf("mirrorlist url %q must be http or https", u)
+		}
+	}
+	return nil
+}
+
 // ValidatePatterns ensures every configured regex compiles. Storing an
 // invalid pattern would otherwise be silently dropped at match time, which
 // for the blocklist is a fail-open: a mistyped deny rule becomes a no-op.