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 every webhook's database. 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 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)
 	}
 }