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/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,