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 +699,9 @@ func (e *Engine) recoverInFlight(ctx context.Context) {
 		default:
 		}
 
-		if !e.dbManager.DBExists(webhookID) {
-			continue
-		}
-
+		// Opened even when its file is missing, so that a lost
+		// database is reported at start, not when the webhook next
+		// receives an event, which for a quiet webhook may be never.
 		e.recoverWebhookDeliveries(ctx, webhookID)
 	}
 }
@@ -710,7 +709,24 @@ func (e *Engine) recoverInFlight(ctx context.Context) {
 func (e *Engine) recoverWebhookDeliveries(
 	ctx context.Context, webhookID string,
 ) {
-	webhookDB, err := e.dbManager.GetDB(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.
+	stillExists := func() (bool, error) {
+		var count int64
+
+		err := e.database.DB().
+			Model(&database.Webhook{}).
+			Where("id = ?", webhookID).
+			Count(&count).Error
+		if err != nil {
+			return false, fmt.Errorf("confirming webhook exists: %w", err)
+		}
+
+		return count > 0, nil
+	}
+
+	webhookDB, err := e.dbManager.GetDBIf(webhookID, stillExists)
 	if err != nil {
 		e.log.Error(
 			"failed to get webhook database for recovery",
@@ -721,6 +737,10 @@ func (e *Engine) recoverWebhookDeliveries(
 		return
 	}
 
+	if webhookDB == nil {
+		return
+	}
+
 	e.recoverPendingDeliveries(
 		ctx, webhookDB, webhookID,
 	)