Give a second snapshot create in the same second its own ID (closes #270)

The timestamp in a snapshot ID is in whole seconds, so a `snapshot
create` that started in the same second as the previous run of that
snapshot name got the same ID, and inserting its row failed with
`UNIQUE constraint failed: snapshots.id`. CreateSnapshotWithName now
looks the ID up in the local index first and, if it is taken, waits a
second and takes a new timestamp. The ID format is unchanged.

Only the local index is checked. That is where the insert fails, and
the process lock serializes runs, so nothing takes the ID between the
lookup and the insert.

Model: opus-5-5

internal/snapshot/snapshot.go

@@ -115,20 +115,37 @@ func ShortHostname(hostname string) string {
 
 // CreateSnapshotWithName creates a new snapshot record with an optional
 // snapshot name. The snapshot ID format is: hostname_name_timestamp or
-// hostname_timestamp if name is empty.
+// hostname_timestamp if name is empty. The timestamp is in whole seconds.
+// If the local index already has a snapshot with that ID, from a run of the
+// same name that started in the same second, it waits a second and takes a
+// new timestamp.
 func (sm *SnapshotManager) CreateSnapshotWithName(
 	ctx context.Context, hostname, name, version, gitRevision string,
 ) (string, error) {
 	shortHostname := ShortHostname(hostname)
 
-	// Build snapshot ID with optional name
-	timestamp := time.Now().UTC().Format("2006-01-02T15:04:05Z")
-
 	var snapshotID string
-	if name != "" {
-		snapshotID = fmt.Sprintf("%s_%s_%s", shortHostname, name, timestamp)
-	} else {
-		snapshotID = fmt.Sprintf("%s_%s", shortHostname, timestamp)
+
+	for {
+		// Build snapshot ID with optional name
+		timestamp := time.Now().UTC().Format("2006-01-02T15:04:05Z")
+
+		if name != "" {
+			snapshotID = fmt.Sprintf("%s_%s_%s", shortHostname, name, timestamp)
+		} else {
+			snapshotID = fmt.Sprintf("%s_%s", shortHostname, timestamp)
+		}
+
+		existing, err := sm.repos.Snapshots.GetByID(ctx, snapshotID)
+		if err != nil {
+			return "", fmt.Errorf("looking up snapshot %s: %w", snapshotID, err)
+		}
+
+		if existing == nil {
+			break
+		}
+
+		time.Sleep(time.Second)
 	}
 
 	snapshot := &database.Snapshot{