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

An audit of every file webhooker reads configuration or required state from found cases that carried on silently. A zero-length webhooker.db, and a missing or zero-length per-webhook database, now log the "created a new, empty database" warning naming the file; restart recovery opens every live webhook's database, checking under the manager's lock that it still exists, so a missing one is reported at start. The main database's open errors name webhooker.db, for the server and webhooker resetpw; resetpw refuses a zero-length webhooker.db. A directory in place of any database file or its -wal or -shm is refused naming it. The README says how each case is treated. Also closes #459.

Model: opus-5-5

internal/delivery/terminal_state_test.go

@@ -573,6 +573,8 @@ func TestRecoverPending_TargetDeleted(t *testing.T) {
 
 	s := newISetup(t)
 
+	iCreateWebhook(t, s.MainDB, s.WebhookID, "pending-recovery")
+
 	deliveryID := tSeedDeletedTarget(
 		t, s, "gone-while-pending", "http://example.com/hook",
 		database.DeliveryStatusPending,
@@ -612,6 +614,8 @@ func TestRecoverPending_TargetDeleted_LeavesAnOwnedDeliveryAlone(
 
 	s := newISetup(t)
 
+	iCreateWebhook(t, s.MainDB, s.WebhookID, "owned-recovery")
+
 	deliveryID := tSeedDeletedTarget(
 		t, s, "gone-but-owned", "http://example.com/hook",
 		database.DeliveryStatusPending,