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>

internal/database/remotes.go

@@ -6,7 +6,7 @@ import (
 	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
-const remoteCols = `name, package_type, repo_type, base_url, description, username, password,
+const remoteCols = `name, package_type, repo_type, base_url, mirrorlist, description, username, password,
 	immutable_ttl, mutable_ttl, check_mutable,
 	patterns, blocklist, mutable_patterns, immutable_patterns,
 	ban_tags_enabled, ban_tags,
@@ -17,7 +17,7 @@ const remoteCols = `name, package_type, repo_type, base_url, description, userna
 
 func scanRemote(scanner interface{ Scan(...any) error }, r *models.Remote) error {
 	return scanner.Scan(
-		&r.Name, &r.PackageType, &r.RepoType, &r.BaseURL, &r.Description, &r.Username, &r.Password,
+		&r.Name, &r.PackageType, &r.RepoType, &r.BaseURL, &r.Mirrorlist, &r.Description, &r.Username, &r.Password,
 		&r.ImmutableTTL, &r.MutableTTL, &r.CheckMutable,
 		&r.Patterns, &r.Blocklist, &r.MutablePatterns, &r.ImmutablePatterns,
 		&r.BanTagsEnabled, &r.BanTags,
@@ -58,16 +58,16 @@ func (db *DB) ListRemotes(ctx context.Context) ([]models.Remote, error) {
 func (db *DB) CreateRemote(ctx context.Context, r *models.Remote) error {
 	_, err := db.Pool.Exec(ctx, `
 		INSERT INTO remotes (
-			name, package_type, repo_type, base_url, description, username, password,
+			name, package_type, repo_type, base_url, mirrorlist, description, username, password,
 			immutable_ttl, mutable_ttl, check_mutable,
 			patterns, blocklist, mutable_patterns, immutable_patterns,
 			ban_tags_enabled, ban_tags,
 			quarantine_enabled, quarantine_days, stale_on_error,
 			releases_remote, managed_by,
 			upstream_dial_timeout, upstream_tls_timeout, upstream_response_header_timeout
-		) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24)
+		) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24,$25)
 	`,
-		r.Name, r.PackageType, r.RepoType, r.BaseURL, r.Description, r.Username, r.Password,
+		r.Name, r.PackageType, r.RepoType, r.BaseURL, r.Mirrorlist, r.Description, r.Username, r.Password,
 		r.ImmutableTTL, r.MutableTTL, r.CheckMutable,
 		r.Patterns, r.Blocklist, r.MutablePatterns, r.ImmutablePatterns,
 		r.BanTagsEnabled, r.BanTags,
@@ -81,7 +81,7 @@ func (db *DB) CreateRemote(ctx context.Context, r *models.Remote) error {
 func (db *DB) UpdateRemote(ctx context.Context, r *models.Remote) error {
 	_, err := db.Pool.Exec(ctx, `
 		UPDATE remotes SET
-			package_type=$2, repo_type=$3, base_url=$4, description=$5, username=$6, password=$7,
+			package_type=$2, repo_type=$3, base_url=$4, mirrorlist=$25, description=$5, username=$6, password=$7,
 			immutable_ttl=$8, mutable_ttl=$9, check_mutable=$10,
 			patterns=$11, blocklist=$12, mutable_patterns=$13, immutable_patterns=$14,
 			ban_tags_enabled=$15, ban_tags=$16,
@@ -98,6 +98,7 @@ func (db *DB) UpdateRemote(ctx context.Context, r *models.Remote) error {
 		r.QuarantineEnabled, r.QuarantineDays, r.StaleOnError,
 		r.ReleasesRemote, r.ManagedBy,
 		r.UpstreamDialTimeout, r.UpstreamTLSTimeout, r.UpstreamResponseHeaderTimeout,
+		r.Mirrorlist,
 	)
 	return err
 }