rpm: make local repodata deterministic (fixes #117) (#118)

Local RPM repos regenerated `repomd.xml` on every request and advertised a `primary.xml.gz` sha256 that drifted every second, because `time.Now().Unix()` was embedded inside the gzipped `primary.xml` (and in `repomd` `<revision>`/`<timestamp>`). The advertised hash therefore never matched the content-addressed `<sha256>-primary.xml.gz` bytes a second later or on the other replica, so `dnf` failed with a checksum mismatch. Part of #117.

How:
- Derives `<time file=>` in `primary.xml` from the persisted `rpm_metadata.created_at` instead of the wall clock; unset timestamps collapse to a fixed `0`.
- Derives `repomd` `<revision>`/`<timestamp>` from the newest package upload time, so `repomd.xml` is byte-identical across replicas and requests.
- Adds `file_path` as a total-order tiebreak to the metadata `ORDER BY`.
- Pins the gzip header (`OS: 255`) so compressed bytes depend only on the payload.
- Adds regression tests: generators are byte-identical across two runs, and the sha256 in `repomd.xml` equals the sha256 of the bytes each `serve*` handler returns.

Reviewed-on: #118
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/provider/provider.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"time"
 
 	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
@@ -185,6 +186,9 @@ type RPMMetadata struct {
 	Obsoletes     []RPMDep
 	Files         []RPMFile
 	Changelogs    []RPMChangelog
+	// CreatedAt is the persisted upload timestamp; used as a stable, replica-independent
+	// value for the repodata <time>/<revision> fields so generated indexes are deterministic.
+	CreatedAt time.Time
 }
 
 type RPMDep struct {