feat: server-level GitHub machine credential for authenticated requests (#109)

## Why

Anonymous GitHub is capped at 60 requests/hour and cannot read private repositories. A machine credential usable by a free (non-enterprise) account is needed to lift the request budget to ~5000/hr and to read private-repo release assets.

Builds on the background syncer (#108, now merged to `master`); this diff is the auth changes only.

## How

- Add `internal/githubauth`: a process-wide GitHub credential delivered via env/secret, applied by default to every outbound GitHub request (releases scan, ranged asset-header GETs, and the generic-github byte proxy for private assets).
- Support two modes:
  - **PAT** — `GITHUB_TOKEN` sent as `Authorization: Bearer <token>`.
  - **GitHub App** — `GITHUB_APP_ID` + `GITHUB_APP_INSTALLATION_ID` + private key (`GITHUB_APP_PRIVATE_KEY` inline PEM or `GITHUB_APP_PRIVATE_KEY_PATH`). Mint a short-lived RS256 JWT with stdlib `crypto/rsa` (no new dependency), exchange it at `POST /app/installations/{id}/access_tokens` for a ~1h installation token, cache it, and single-flight a refresh a few minutes before expiry.
- Inject at the two GitHub call paths: the rpm github provider header builder (releases + ranged fetches) and the generic provider `AuthHeaders` (byte proxy, github.com hosts only; the pre-signed `objects.githubusercontent.com` redirect deliberately gets no Authorization).
- Honor precedence: a remote's own `username`/`password` overrides the server credential; no credential configured stays anonymous (current behavior).
- Fail closed at startup on partial App configuration (e.g. App id without a private key); a token-and-App conflict is also rejected.
- Never persist the credential to the DB, return it from an API, or log it (token-exchange failures never echo the response body).
- Read config via the existing `getenv` convention; document PAT vs App setup, the free-account fine-grained PAT scopes (Contents:read + Metadata:read), precedence, and the rate-limit implication.

## Rate limit

Authenticated requests share the syncer's single global limiter — no second limiter is added. A token raises the effective GitHub ceiling (~5000/hr vs ~60/hr), so the limiter defaults stay safe.

## Tests

`internal/githubauth` and `internal/provider/{rpm,generic}`:
- PAT attaches the correct `Authorization` header to releases + asset-header requests.
- App mints a valid RS256 JWT (verified against the app public key), exchanges it at a mocked endpoint, reuses the cached token without re-exchanging, refreshes near expiry, and single-flights concurrent callers.
- Per-remote credential overrides the server credential (rpm + generic).
- No credential → no `Authorization` header, requests still succeed anonymously.
- ETag/304 flow still works with auth attached.
- The credential does not appear in a remote's serialized JSON.
- Config validation: no-config is anonymous; partial App config and token/App conflict both error.

Verified fail-before/pass-after for the injection tests. `gofmt -l`, `go build ./...`, `go vet ./...`, `go test ./...` all clean (26 packages).

Reviewed-on: #109
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/config/env.go

@@ -44,6 +44,18 @@ type Config struct {
 	GitHubSyncBurst        int
 	GitHubSyncWorkers      int
 	GitHubSyncPollInterval int
+
+	// Server-level GitHub machine credential, applied by default to every
+	// outbound GitHub request (releases scan, ranged asset fetches, and the
+	// generic-github byte proxy for private assets). Delivered via env/secret
+	// only — never stored per-remote, never returned by an API, never logged.
+	// Configure exactly one mode: a Personal Access Token, or a GitHub App
+	// (id + installation id + private key). Partial App config fails at startup.
+	GitHubToken             string
+	GitHubAppID             string
+	GitHubAppInstallationID string
+	GitHubAppPrivateKey     string
+	GitHubAppPrivateKeyPath string
 }
 
 func (c *Config) DatabaseDSN() string {
@@ -105,6 +117,12 @@ func Load() (*Config, error) {
 		GitHubSyncBurst:        syncBurst,
 		GitHubSyncWorkers:      syncWorkers,
 		GitHubSyncPollInterval: syncPoll,
+
+		GitHubToken:             getenv("GITHUB_TOKEN", ""),
+		GitHubAppID:             getenv("GITHUB_APP_ID", ""),
+		GitHubAppInstallationID: getenv("GITHUB_APP_INSTALLATION_ID", ""),
+		GitHubAppPrivateKey:     getenv("GITHUB_APP_PRIVATE_KEY", ""),
+		GitHubAppPrivateKeyPath: getenv("GITHUB_APP_PRIVATE_KEY_PATH", ""),
 	}
 
 	return cfg, nil