Serve GitHub code scanning and Dependabot alerts per snapshot

GET /api/snapshots/{id}/alerts fetches open code scanning alerts
(located at the flagged file and line) and open Dependabot alerts
(located at the vulnerable manifest) for the snapshot's repository,
using the viewer's own GitHub token, in parallel and following Link
pagination only while it stays on the API host. Severities are
normalised to critical/high/medium/low. Each set reports ok, forbidden
(permission not granted or feature disabled) or unavailable; signed-out
viewers and local folders get a reason instead. Snapshots now remember
their GitHub repository URL.

indexer.go

@@ -48,6 +48,7 @@ type Edge struct {
 type Snapshot struct {
 	ID, Name, Source string
 	Owner            string // session that created the snapshot
+	Repo             string // GitHub repository URL, empty for local folders
 	Files            []FileRecord
 	Edges            []Edge
 	TotalLines       int