Fail GitHub scans on asset errors and drop sync state with its remote (#135)

A GitHub release scan that hit a transient asset error still saved the release-list ETag, so the failed asset was never retried. Assets that are corrupt, deleted or truncated upstream fail on every retry, so they must not keep the remote failed.

- fail the rpm/deb/alpine scan on transient asset fetch, short read or metadata insert errors, so the ETag is not saved and the backoff retry re-derives the asset
- add `provider.ErrInvalidPackage` for unparseable packages and 404/410/416 asset downloads; such assets are logged and skipped
- decide a ranged fetch holds the whole object from Content-Length or the Content-Range total, or a clean EOF when the total is unknown
- add a NOT VALID cascading FK from the GitHub sync-state tables to `remotes`

Reviewed-on: #135
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

migrations/0003_github_sync_state_cascade.sql

@@ -0,0 +1,12 @@
+-- Sync state is dropped with its remote, so a recreated remote of the same name
+-- starts with no inherited ETag or timing. NOT VALID skips checking rows left by
+-- remotes deleted before this migration, which would otherwise fail it.
+ALTER TABLE github_rpm_sync_state    DROP CONSTRAINT IF EXISTS github_rpm_sync_state_remote_fk;
+ALTER TABLE github_rpm_sync_state    ADD CONSTRAINT github_rpm_sync_state_remote_fk
+	FOREIGN KEY (remote_name) REFERENCES remotes(name) ON DELETE CASCADE NOT VALID;
+ALTER TABLE github_deb_sync_state    DROP CONSTRAINT IF EXISTS github_deb_sync_state_remote_fk;
+ALTER TABLE github_deb_sync_state    ADD CONSTRAINT github_deb_sync_state_remote_fk
+	FOREIGN KEY (remote_name) REFERENCES remotes(name) ON DELETE CASCADE NOT VALID;
+ALTER TABLE github_alpine_sync_state DROP CONSTRAINT IF EXISTS github_alpine_sync_state_remote_fk;
+ALTER TABLE github_alpine_sync_state ADD CONSTRAINT github_alpine_sync_state_remote_fk
+	FOREIGN KEY (remote_name) REFERENCES remotes(name) ON DELETE CASCADE NOT VALID;