Add github_deb metadata-only package type (#112)

## Why

This stacks the Debian/apt analog of `github_rpm` on top of the deb local+remote work (#111). It lets a GitHub repo's `.deb` release assets be consumed as a real apt repository without artifactapi ever precaching whole packages: it derives per-asset control metadata from a ranged prefix fetch, synthesizes a flat apt repo from the cache, and redirects the actual `.deb` downloads to a backend `releases_remote` (the generic github.com remote).

Base is `benvin/deb-local-remote` (stacked) to keep the diff atomic.

## How

- Adds `github_deb` to the package-type enum and validity map.
- Adds the `github_deb` provider mirroring `github_rpm`: `ServeRemote` serves `Packages`/`Packages.gz`/`Release`, returns 404 for `InRelease`/`Release.gpg` (unsigned, consumed via `[trusted=yes]`), and 302-redirects `*.deb` to `{proxyBaseURL}/api/v1/remote/{releases_remote}/{path}`; cold-start prime with a retryable 503.
- `deriveAsset` ranged-GETs the front of the `.deb` (an `ar` archive), locates and fully reads `control.tar.*`, and parses the control paragraph — doubling the range if the control member is truncated. The Packages `SHA256` comes from the GitHub asset `digest` when present, else a one-time full stream; `MD5sum` is left unset (apt verifies against SHA256 under `[trusted=yes]`).
- Adds a `github_deb` background Syncer (own worker pool, shared rate limiter, deduped queue) with per-remote DB-lease-gated scans so only one replica scans per window.
- Adds the `github_deb_sync_state` table plus `ListGitHubDebRemotes` / `ClaimGitHubDebSyncLease` / `ReleaseGitHubDebSyncLease` DB helpers, kept separate from the rpm ones.
- Primes `github_deb` remotes on create and runs the deb syncer alongside the rpm one; prime-on-create is routed by package type.
- Reuses the deb apt-index generators and control parser; the Packages generator now skips empty hash lines so a SHA256-only entry is valid.

## Notes / deviations

- **Filename convention:** the `Filename` stored in the Packages index is the **github-relative** asset path (same as rpm's `assetPath`), not `pool/<asset>`. This is required for the `.deb` 302 to `{releases_remote=github}/{path}` to resolve against github.com; it still matches the `*.deb` redirect rule.
- **GitHub client helpers** (releases pagination, ranged GET, auth headers) are duplicated into the deb package rather than shared, because the rpm equivalents are unexported in `package rpm` and the task requires not modifying the rpm provider.
- `go build`, `go vet`, `go mod tidy`, and `make test` (`-race`, incl. the Postgres lease integration tests) all pass; pre-commit clean.

Do not merge — for review.

---------

Co-authored-by: unkin-agent <unkin-agent@git.unkin.net>
Reviewed-on: #112
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/database/deb_github_sync.go

@@ -0,0 +1,70 @@
+package database
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+// ListGitHubDebRemotes returns every github_deb remote so the syncer can sweep
+// them on each poll tick.
+func (db *DB) ListGitHubDebRemotes(ctx context.Context) ([]models.Remote, error) {
+	rows, err := db.Pool.Query(ctx, `SELECT `+remoteCols+` FROM remotes WHERE package_type = $1 ORDER BY name`, models.PackageGitHubDeb)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var remotes []models.Remote
+	for rows.Next() {
+		var r models.Remote
+		if err := scanRemote(rows, &r); err != nil {
+			return nil, err
+		}
+		remotes = append(remotes, r)
+	}
+	return remotes, rows.Err()
+}
+
+// ClaimGitHubDebSyncLease atomically claims the per-remote sync lease. It
+// succeeds only when the remote is due (never synced, or synced longer than
+// freshness ago) and no live lease is held by another replica. A zero freshness
+// (prime scans) ignores the recency gate. The returned etag is the stored
+// releases-list ETag, shared across replicas.
+func (db *DB) ClaimGitHubDebSyncLease(ctx context.Context, remoteName, owner string, freshness, lease time.Duration) (bool, string, error) {
+	row := db.Pool.QueryRow(ctx, `
+		INSERT INTO github_deb_sync_state AS s (remote_name, sync_lease_owner, sync_lease_expires)
+		VALUES ($1, $2, now() + make_interval(secs => $4))
+		ON CONFLICT (remote_name) DO UPDATE
+			SET sync_lease_owner   = $2,
+			    sync_lease_expires = now() + make_interval(secs => $4)
+			WHERE (s.last_synced_at IS NULL OR s.last_synced_at < now() - make_interval(secs => $3))
+			  AND (s.sync_lease_expires IS NULL OR s.sync_lease_expires < now())
+		RETURNING s.etag
+	`, remoteName, owner, freshness.Seconds(), lease.Seconds())
+
+	var etag string
+	if err := row.Scan(&etag); err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return false, "", nil
+		}
+		return false, "", err
+	}
+	return true, etag, nil
+}
+
+// ReleaseGitHubDebSyncLease records the completed scan and frees the lease. Only
+// the owning replica may release; last_synced_at advances so the next poll waits
+// a full freshness window, and etag is persisted for the next conditional request.
+func (db *DB) ReleaseGitHubDebSyncLease(ctx context.Context, remoteName, owner, etag string, syncedAt time.Time) error {
+	_, err := db.Pool.Exec(ctx, `
+		UPDATE github_deb_sync_state
+		   SET last_synced_at = $3, etag = $4, sync_lease_owner = '', sync_lease_expires = NULL
+		 WHERE remote_name = $1 AND sync_lease_owner = $2
+	`, remoteName, owner, syncedAt, etag)
+	return err
+}