Report or refuse each unusable file webhooker reads (closes #290)

Audit of the files webhooker reads configuration or required state
from. A missing or zero-length database is reported with the "created
a new, empty database" warning and its path: webhooker.db at start,
and a per-webhook database at the latest at the next start, since
restart recovery now opens the database of every webhook that still
exists. The main database's open errors name webhooker.db
(#459). webhooker resetpw
refuses a zero-length webhooker.db as it refuses a missing one. A
directory in place of a database file or its -wal or -shm is refused,
naming it; beside a -shm directory SQLite opened the database
read-only without a word. The README says how each case is treated.

Model: opus-5-5

internal/delivery/engine.go

@@ -699,10 +699,33 @@ func (e *Engine) recoverInFlight(ctx context.Context) {
 		default:
 		}
 
-		if !e.dbManager.DBExists(webhookID) {
+		// The web interface is already serving, so the webhook may
+		// have been deleted since the list was read. Opening its
+		// database then would create the file again after the delete
+		// removed it.
+		var count int64
+
+		err := e.database.DB().
+			Model(&database.Webhook{}).
+			Where("id = ?", webhookID).
+			Count(&count).Error
+		if err != nil {
+			e.log.Error(
+				"failed to confirm webhook exists for recovery",
+				"webhook_id", webhookID,
+				"error", err,
+			)
+
 			continue
 		}
 
+		if count == 0 {
+			continue
+		}
+
+		// Opened even when its file is missing, so that GetDB reports
+		// a lost database at start, not when the webhook next receives
+		// an event, which for a quiet webhook may be never.
 		e.recoverWebhookDeliveries(ctx, webhookID)
 	}
 }